Cal Thulu is a graduate from Charles Darwin Middle School. He was never seen, but his name was seen on the Charles Darwin book of successful alumni in the episode "Mountain Dude". His species, while unconfirmed it most likely that of the fictional demon, Cthulhu, who would technically qualify as a reptile.
In "Mountain Dude", Mr. Mandrill went through the book of CDMS's successful alumni, and found Dennis Billy. On the other page, the name "Cal Thulu" was seen, above a dark photo with a obscuring glare, partially hiding an inconspicuous and unidentifiable figure, making the student's appearance and species unknown. Due to the intended pun, given from his name, it can be assumed that Cal Thulu is a Cthulhu. It should also be noted that when Principal Pixiefrog questioned if Dennis Billy was really successful or not, Mr. Mandrill said "No, but the other two successful alumni won't speak to me." Cal could have been one of the successful alumni, due to Cthulhu's popularity, so Cal must have some kind of beef with Mr. Mandrill, and refuses to talk to him anymore.