Cosmo (full name Cosmo T.D. "The Dog" Fuller) also briefly known as Derek is a recurring character in Fuller House. He is the pet Golden Retriever of the Fuller family, though his owner is technically Max Fuller. Cosmo is the son of Comet Jr. Jr. and the great-grandson of Comet, the Tanner family's former pet.
The dog who played Cosmo passed away during surgery on December 16th, 2019.