Boxwood

ASPCA lists Boxwood (Buxus spp.) as Toxic to Dogs, Toxic to Cats, AND Toxic to Horses: ASPCA page, whole-genus listing consistent with Clemson's own multi-species treatment. Toxic principle: alkaloids. ASPCA's own Clinical Signs field is already species-differentiated, word for word: "Dogs and cats: vomiting, diarrhea; Horses: colic, diarrhea, respiratory failure, seizures." ASPCA does not itself rank these as more or less severe: it lists signs per species, not a severity ranking. Reading those listed signs, the horse-specific signs (colic, respiratory failure, seizures) are more serious in character than the dog/cat signs (vomiting, diarrhea), which is the own reasonable inference from ASPCA's list, not a comparison ASPCA states explicitly. Worth noting for any user with horses or boarding near a boxwood hedge, not just dog/cat owners. NC State's independent listing agrees on toxicity (steroidal alkaloids, similar GI/neurological symptom profile) and adds that human contact with the plant can also cause contact dermatitis, a detail ASPCA's animal-focused page doesn't address.