From Muppet Wiki

Spotted Dick is one of the mutinous pirates in Muppet Treasure Island. During the "Cabin Fever" number, he is performed by Jerry Nelson. He also appeared in Muppets Tonight episode 203 and reunited with many of his fellow pirates as a Flying Monkey in The Muppets' Wizard of Oz (performed in the latter by Mike Quinn). He resembles a purple crocodile with orange spots.

Spotted dick is also the name of a popular British dessert.