"What did the ___ say when it was riding on the back of a turtle? Wheeeee!" (5)
I believe the answer is:
snail
(Other definitions for snail that I've seen before include "Its speed was emulated by Shakespeare's creeping schoolboy" , "Proverbially slow-moving mollusc" , "Slow mover" , "Sluggish person - kind of mail" , "trailer" .)