Plant Toxicity -- Many animals may intentionally ingest plants that are toxic to them.  This may be done during normal foraging in large animals and out of boredom or curiosity in small animals.  Typically, younger pets are more curious and plant-ingested toxicity is more common in them.  Also changes in the pets surroundings (new plants or a new home) or if they are left alone for an abnormally long period of time, the risk of ingestion is greater.  Obviously, house plants are ornamentals are more commonly encountered in small animal poisonings while wild plants are more commonly the cause of toxicity in large animals.

Many of these plants will produce toxicities similar to those observed in humans.  Following are brief summaries of poisonous plants, their active constituents where known, and the signs of toxicity.
Houseplants/Ornamentals
Yew (Taxus) -- alkaloids, cyanide, oils -- nervousness, trembling, ataxia, dyspnœa, collapse, bradycardia, cardiac standstill, and death without struggle.

Azalea & Rhododendron -- andromedotoxins -- cholinergic effects, convulsions, coma, death.

Philodendron -- oxalates -- vomiting, diarrhœa, excitation, convulsions, and encephalitis in cats.

English Holly (Ilex) -- saponins, alkaloids, triterpenes, glycosides -- GI pain, vomiting, diarrhœa, death

Poinsettia (Euphorbia) -- irritants (?) -- salivation and vomiting

Shooting Star (Cyclamen) -- triterpenoids -- GI upset, convulsions

Angel Wings (Caladium) -- oxalates -- GI upset

Spider or Airplane Plant (Chlorophytum) -- alkaloids (?) -- vomiting, retching, salivation, diarrhœa.

Dumbcane (Dieffenbachia) -- oxalates -- GI upset

Mother-in-Law Tongue (Sansaveria) -- saponins -- vomiting, diarrhœa, salivation, hæmolysis

Schefflera -- oxalates -- GI upset

Wild Plants -- this summary also notes the species that most commonly ingest the plant.
Pokeweed -- swine -- oxalates -- vomiting, diarrhœa, convulsions, respiratory failure, death

Oaks -- cattle -- gallotannin -- anorexia, constipation progressing to dark, tarry diarrhœa, death

Cocklebur -- swine -- carboxyatractyloside -- anorexia, nausea, vomiting, convulsions

Buckeye -- all -- glycoside -- CNS depression, twitching, paralysis

Skunk cabbage -- swine, cattle, sheep -- steroids, alkaloids -- vomiting, digitalis-like effects, congenital cyclops in lambs.

Red Maple -- horses -- ? -- methæmoglobinæmia, hæmolysis, icterus, cyanosis

White Snake Root -- swine, cattle, horses -- benzyl alcohol -- hepatotoxicity, throat paralysis, death within 1-3 days.

Black Locust -- horses -- ? -- anorexia, diarrhœa, weakness, posterior paralysis, laminitis, long recovery

Black Walnut -- horses -- ? -- CNS depression, hyperthermia, tachypnœa, tachycardia, laminitis

Chinaberry -- swine, sheep, other species are less susceptible to toxicity -- vomiting, constipation, cyanosis, tachycardia, dyspnœa, death within 24 hr.

Milkweed -- all -- ? -- convulsions, mydriasis, coma, death

Milkvetch -- all -- ? -- posterior paralysis, goose-stepping, spinal cord demyelination, death

Johnson Grass, Sorghum, Milo, and Broomcorn -- all -- The causative agents differ depending upon the growth of the plant.  If there are drought conditions and if the plants have been trampled or subjected to frost the primary toxicant is hydrocyanic acid, producing bright red blood.  If there are drought conditions and the area has been heavily fertilised, the toxicants are nitrates, producing a chocolate brown blood.  Other signs of toxicity with either growth include dyspnœa, bloat, staggering, convulsions, and death.

Mycotoxins
Many fungi may cause mycotoxicoses.  These plants often grow on older or spoiling food sources and may present as different syndromes.
Œstrogenism -- Fusarium spp. of fungus produce zearalenone, a naturally occuring œstrogen.  It is commonly found on corn and cereal feeds.  It produces vulvovaginitis in pigs, uterine hypertrophy and vaginal discharge in cattle, and decreased egg production in poultry.

Ergotism (Paspalum Staggers) -- Claviceps fungi (paspalinine) commonly grow on grasses and grains.  It produces tremors, jerky movements, ataxia, and falling down.

Aflatoxicosis -- The aflatoxins (from Aspergillus spp.) may be produced on mold feed either in the feild or in storage.  Common sources include peanuts, soybeans, rice, sorghum, corn, and cereals.  Aflatoxicosis will exhibit as unthriftiness, weakness, anorexia, and sudden death.  If the animal recovers, hepatotoxicity often leads to late death.  An antidote for aflatoxicosis is hydrated sodium calcium aluminosilicates, which adhere to the aflatoxin, preventing it from exerting its toxic action.

Slobbers -- Slaframine poisoning from Rhizoctonia, affects primarily sheep and cattle.  Sources commonly include legumes, especially red clover.  Slobbers presents as salivation, bloat, diarrhœa, and death.

Fescue Foot -- Macrocyclic trichothecenes poisoning from Myrothecium spp., affects cattle.  The source is tall fescue grass.  Fescue foot presents as lameness, weight loss, hyperthermia, and dry gangrene of the extremeties.

Sweet Clover Poisoning -- Dicoumarol toxicity from Penicillium spp. that grows on spoiling sweet clover.  Affected species are most often cattle, horses, and sheep.  The animals present with hæmorrhage, stiffness, lameness (due to bleeding into the muscles and joints), hæmotoma, epistaxis, and GI bleeding.  Treatment is vitamin K.  The animals may also require whole blood transfusion.

Snake bites can be deadly, however treatment is similar that for human bite victims.

A more common source of animal poisonings in small animals is toad poisoning in dogs and, less often, in cats.  The severity of the toxicity depends upon the extent of contact and the type of toad.  The toxins (bufagins, bufatoxins, catecholamines, serotonin) are secreted by glands dorsal and posterior to the eyes and the secretion is typically a thick, creamy, white substance.  Exposure most often occurs when the animal attempts to bite or pick up the toad, although poisoning may occur with sniffing or licking.  The poisoned animal will present with profuse, frothy salivation, vigorous head shaking, pawing at the mouth, retching, vomiting, seizures, cardiac arrhythmias, and potential death.  Initial treatment is to wash the mouth of the animal thoroughly.  Atropine may be administered as a physiologic antidote.  Symptomatic treatment and supportive care is often required to ensure survival.

Iron Toxicity in Newborn Pigs -- Oftentimes newborn piglets are given a SC/IM injection of iron.  Injuring around the site of injection or a poorly administered injection may cause local potassium release and cardiotoxicity.  Additionally, the iron may be rapidly distributed as a result of the injury, causing damage to phagocytes (leading to infection and potential death).  The iron may also cause calcium mobilisation, resulting in excess calcium (calciphylaxis) and death.

Lead Poisoning -- Grazing animals may experience lead toxicity resulting from contaminated pastures (from junk metal, old batteries, et c.).  Retrieving dogs have also experiences lead toxicity from the shot in game.  Severe lead toxicity will present with bellowing, staggering, manical excitement, crashing into objects, the appearance of blindness, ataxia, circling, leaning, muscle twitches, eyeball snapping, and convulsions.  Death may occur as quickly as within 2 hours.  The treatment for lead poisoning is a cathartic to remove any un-absorbed lead and calcium EDTA to chelate to circulating lead.

Salt Poisoning -- In cases where there is water deprivation and excess salt intake, salt toxicity may occur, most commonly in sheep.  While 1% NaCl in water is permissible for sheep, 1.5% may prove fatal.  Sheep will present with blindness, deafness, wandering, pushing, circling, and seizures.  Cattle often present with anorexia, vomiting, diarrhœa, abdominal pain, mucous in the fæces, dragging their hind feet, and knuckling of the fetlock.  The treatment is water (slow administration in small volumes -- too rapid administration will increase the neurologic signs of toxicity and increase cerebral œdema).  Even with treatment, the mortality rate for salt toxicity is approximately 50%.

Ethylene Glycol -- The active ingredient in many older antifreezes, ethylene glycol toxicity occurs primarily in dogs and cats.  They will drink the antifreeze, even if water is available, perhaps because of its sweetness.  The toxicity is primarily a result of the metabolic byproducts glycolate and oxalate.  The median lethal dose undiluted is 1.5 ml/Kg in cats and 6.6 ml/Kg in dogs.  The MLD diluted is 15 ml in cats and 130 ml in dogs.  Signs of acute toxicity include polydipsia, polyuria, knuckling, ataxia, vomiting, dehydration, CNS depression, tachycardia, and tachypnœa.  Signs of subacute toxicity, secondary to renal failure, include CNS depression, anorexia, gastroenteritis, encephalopathy, coma, and death.  The treatment for ethylene glycol toxicity is ethanol or 4-methylpyrazole.  The former competes with EG for binding sites on alcohol dehydrogenase while the latter inhibits alcohol dehydrogenase.  Both actions result in a decrease in the formation of toxic metabolites and the enhance excretion of unchanges EG.  If the animal is in failure, supportive care with hydration and diuresis should be provided, allowing the kidney to recover (if irreversible damage has not already occurred).

Chocolate -- Small animals, especially dogs, are extremely sensitive to theobromine, the primary active alkaloid in chocolate.  Ingestion of as little as 4/10 oz. milk chocolate/Kg body weight may show signs of toxicity.  Ten times this dose may be fatal.  Signs of toxicity include nervousness, restlessness, insomnia, tremors, clonic seizures, tachypnœa, nausea, and vomiting.  The presentation of signs and symptoms of toxicity is unique to each animal.  There may be a delay of the onset of symptoms (as much as 8 hours post ingestion) or there may be no signs at all.  Some animals may die abruptly (due to cardiac failure) without exhibiting any signs of toxicity.  Treatment include emesis if ingestion is recent, activated charcoal, cathartics even 6-8 hours post ingestion, and supportive/symptomatic care.