For the reptile-like humanoids, please see Reptilian.
Reptiles were a class of vertebrates, most of which were cold-blooded. Most of them laid eggs and never returned to care for them. (VOY: "Parturition")
In 2154, while suffering from the effects of a Xindi neural parasite, Hoshi Sato asked Jonathan Archer to make sure the R&R she received after returning to Earth was in a location where there were no reptiles around. (ENT: "Zero Hour")
The self-aware machine Vaal from the planet Gamma Trianguli VI resembled a giant reptile with massive fangs and a horn on its nose. (TOS: "The Apple")
A similar reptile was depicted on the communicators used by the terrorists who tried to steal trilithium resin from the USS Enterprise-D in 2369. (TNG: "Starship Mine")
The tribble homeworld of Iota Geminorium IV was home to a large number of reptiles. According to Phlox, they were instrumental in keeping the tribble population in check. (ENT: "The Breach")