It is released during The Purge and can be seen on a computer monitor approaching a woman from behind, about to attack and kill her.
It is possible that a reptile claw summons it.
Reptilius looks to be an homage to the reptile like alien in Species though the most famous reptilian-humanoid from the horror genre is The Creature from the Black Lagoon. It may be based on in slight appearance and demeanor to the Xenomorphs from the Alien franchise. The shot of Reptilius stalking the woman seen on security screens during the purge is reminiscent of when Velociraptors stalk the children through the kitchen in the first Jurassic Park film. The 1978 film, Slithis may also be an inspiration for this half-reptile / half-human creature.
Reptilius may also be based on the mysterious Reptilians aliens, who are said to be responsible for various UFO incidents.