Alligator mississippiensis

Not venomous at all, just enormously strong and shockingly fast for a short distance. Never swim in fresh water at dusk in Florida, and never ever feed one.
A mother alligator carries her babies to the water in her mouth, very gently, without hurting them at all.
Treat it with respect, keep your distance, and tell a grown-up if you see one up close.
Always ask a grown-up before you touch any animal or plant. If something bites or stings you, tell an adult right away.
Source: Florida FWC
Read more about the american alligator at Wikipedia.