A Calrissian chameleon was a reptilian animal capable of changing the color of its scales to blend in with the local environment. Its pituitary gland was not very different than that of a beagle. It secreted a rare toxin that was very useful for treating respiratory infections.
Doctor Phlox kept a female Calrissian chameleon among his menagerie aboard the Enterprise until 2152, when he needed to use its pituitary gland to save the captain's pet, Porthos. Phlox joked that after the surgery, Porthos would be able to blend in with his environment. (ENT: "A Night in Sickbay")