Hylonomus ( /haɪˈlɒnəməs/; hylo- "forest" + nomos "dweller") was a very early reptile. It lived 312 million years ago during the Late Carboniferous period. It is the earliest unquestionable reptile (Westlothiana is older, but may in fact be an amphibian, and Casineria is rather fragmentary). Hylonomus was 20 centimeters (8 inches) long including the tail and probably would have looked rather similar to modern lizards. It had small sharp teeth and ate small invertebrates such as millipedes or early insects.

Hylonomus in The Land Before Time

Hylonomus appears in the first The Land Before Time movie. It lived before the dinosaurs in real life but is shown living with them in the film and it has a sail. When Littlefoot's egg rolls onto the ground, a group of Hylonomus runs away in fear. Two other individuals get close to the egg and smell it but run away scared as it cracks. Hylonomus are also seen witnessing baby Littlefoot after his birth. A curious hatchling even approaches him. They were also seen fleeing during the Great Earthshake. Hylonomus makes cameos in many other The Land Before Time sequels, mostly as background fauna.In The Land Before Time VI: The Secret of Saurus Rock, Littlefoot wakes up a sleeping Hylonomus during his song "On Your Own", causing it to run away. It is a Sailbacked lizard.