The Mosquito. Can make sweet love - orgasm included - in a couple of seconds. (Rabbits and rats also have some quick sex, so I guess depending on the amount of foreplay and lotion, either of these could be correct answers also.)
That was easy. As a bonus, here are a few other fun facts about the animal kingdom.
Flamingos have life long gay partners.
Blue whales ejaculate about eight gallons of gak.
Female porcupines masturbate with sticks.
Squirrel monkeys urinate in each other's faces during mating season.
Pigs have thirty minute orgasms.
Female lions have the most orgasms when in heat.
Flatworms 'penis fight'. Stabbed loser gets to be female.
Hyena ladies basically have a penis.
Dolphins have prehensile retractable penises.
Pandas get to watch panda porn.
Whiptail lizards do lesbian stuff, which prompts egg growth and then they clone themselves.
The Argentine Lake Duck has a penis the size of its whole body and can 'lasso' a mate.
Short Answer: The banana slug's penis is also the length of its body. If it picks the wrong sized mate, its penis gets stuck and the other slug will chew it off. How's that for a short answer?