Final answer:
Oleander and Yew are the toxic plants that can poison a horse with just a single mouthful due to their highly poisonous compounds affecting cardiac function.
Step-by-step explanation:
The toxic plants that are exceptions, as a horse can be poisoned by consuming just a single mouthful, are Oleander and Yew. These plants are highly poisonous to horses, and even a small amount can cause severe cardiac damage or death. Oleander contains cardiac glycosides, which can cause heart failure, while yew contains taxine alkaloids that can rapidly affect cardiac function leading to death.