This article is from the Canine Medical Information FAQ, by Cindy Tittle Moore with numerous contributions by others.
Bufo toads. Found in various areas, especially in south Florida. Very
poisonous -- it can kill a small dog in a matter of minutes. It burns
the mucous membrane of the mouth (gums) which is why they drool and
foam, and that's also how it enters the bloodstream. It kills by
elevating the heart rate and blood pressure to deadly levels, similar
to the effects of chocolate. There is an antidote and the effects can
be lessened if you immediately flush the dog's mouth with water before
taking it to the vet.