He is a dark purple blob that looks like SpongeBob and he was formed from Super Goo.
He was made of Super Goo, a highly toxic substance that was contained in purple goo. Patrick had made him when he got bored and was waiting to get rescued. When he was rescued, he was not adamant about leaving SpongeBlob.
SpongeBlob is similar to Pillow SpongeBob and a Krabby Patty in that Patrick just formed SpongeBob from Super Goo.