The Purple Leech Creature is a character who appears in the episode "Something Smells."
The creature appears to be an alien-like leech that is light purple and has an antenna with a red dot on the top. It may be a parasitic creature who lives off SpongeBob as its host. In that case, it'd be similar to a poison sea urchin.
In "Something Smells," it is used as a part of a joke when Patrick says, "SpongeBob, you're never going to feel better unless you get this thing off your chest."