An egg-laying species, they nested their eggs on a world nicknamed "Planet Hell", which contained vapors that their young fed on.
While on a self-appointed mission to this planet, in 2372, Tom Paris and Neelix inside a cave discovered a nest of eggs, one of which soon hatching a newborn reptohumanoid. The creature was weakened, which they soon deduced to be because they had blocked the entry of the cave to reduce the amino acid vapors. They took the baby out from the cave, and the baby began to regain its strength. After bonding over their concern for the welfare of the creature, they refused to be beamed up by USS Voyager until they were sure the creature was safe. (VOY: "Parturition")
In 2376, at least one member of the Reptohumanoid species attended several Tsunkatse matches as an audience member. (VOY: "Tsunkatse")
The name of the species was not established on screen. However, the Star Trek Encyclopedia (3rd ed., p. 407) has an entry on the "reptohumanoid hatchling". It describes it as a reptillian entity of a technologically advanced species that may not be native to planet Hell.