Use the adjective reptilian to describe a reptile: your pet iguana, while also sweet and possibly cuddly, is definitely reptilian.
Crocodiles, lizards, and turtles are all reptiles, and you can therefore describe all of them as reptilian. A reptilian animal is not a mammal or a bird, and it is most likely cold-blooded and egg laying. You can also use the word reptilian to describe something — or someone — that reminds you of a reptile. The Latin root is reptilis, which means "creeping or crawling."