what do you call an alligator that sneaks up and bites you from behind
a tailgater
A tailgator
The term you are looking for is "ambush predator." An ambush predator is an animal that hides, waits, and then attacks its prey by surprise. In the case of an alligator, it often lurks underwater, partially submerged, waiting patiently until an unsuspecting animal comes close enough for it to strike.
To find this information, you could use an internet search engine like Google. Simply type in "What do you call an alligator that sneaks up and bites you from behind?" and you will likely find the answer in the search results. Additionally, you can consult reputable wildlife or zoology websites to learn more about the hunting behavior of alligators.