Geckos are a type of mutated animal encountered in the wasteland.
Before the war, Geckos were small and harmless lizards. They primarily fed on small insects.
After the war
Geckos have mutated, much like other lifeforms that survived the megaspell and balefire fallout. Geckos are much larger and tend to hunt in groups. They have small sharp teeth and are surprisingly quick. They are carnivorous, feeding on ponies and other creatures they come across.
Though they don't appear in FO:E, they do make appearances in other stories.
A small group attack a pony farmer who is aided by Hired Gun when she bursts onto the scene and helps kill a few of the geckos. They were noted as having poisonous teeth which didn't effect Hired Gun.
Geckos live in and around a vast area around New Pegas. Dead-shot killed a dozen Geckos whilst he was helping a wastelander look for his fillyfriend, Dead-shot was already suspicious of the buck though and killed him when it became obvious it had all been an elaborate trap.
A group of fairly intelligent Geckos, carrying spears and acting like primitive tribals were encountered by Natascha inside a cave which held some old generators. They were driven off after the death of their chieftain.