A good candidate for "the most ridiculously poisonous animal in America": the
rough-skinned newt.
This isn't just a bad frog to lick. The rough-skinned newt produces tetrodotoxin -- the stuff in fugu that kills you if the chef's hands shake. For comparison, potassium cyanide taken orally has an LD50 of 8.5 mg/kg. Tetrodotoxin comes in at an LD50 of 0.3 mg/kg, with serious toxicity at 10 micrograms/kg. It was first discovered when three hunters were found dead around their cooking pot, with an humble-looking newt in it with a bright orange belly.
On the theme of "there's always a bigger fish," however, the local caddis flies and snakes have evolved resistance; the flies will chow down on newt bits given any opportunity, poison or no.
For an additional twist of weirdness, it's hypothesized that a much smaller amount of tetrodotoxin is a potential component of "zombie powder," which an unscrupulous witch doctor might administer to make someone stupefied and compliant.