A reptile kept in Doctor Phlox's sickbay, whose pituitary gland he transplanted into Captain Archer's beagle Porthos to save the dog's life.
The Calrissian chameleon is a reptile kept by Doctor Phlox aboard Enterprise NX-01.
It plays a decisive part in A Night in Sickbay. Porthos picks up a pathogen on the Kreetassan homeworld and his immune system begins to fail. After other treatments have no effect, Phlox proposes a risky operation: replacing the beagle's damaged pituitary gland with one taken from a Calrissian chameleon, whose gland is close enough in function to serve.
The operation is carried out in a tank of fluid with Archer assisting, and Porthos recovers. The long night also forces Archer to face his anger at the Kreetassans, to whom he finally makes the elaborate apology they demand.