Geckos are creatures that were added in Fallout 2 and are going to appear in the upcoming game Fallout: New Vegas. Before the nuclear holocaust of 2077 they were found in warm states or countries around the world, much like the geckos or lizards in our universe. Geckos are only found in the Core Region and throughout the midwest, which is why there are no geckos on the East Coast. They came in a variety of colors, but geckos in Fallout 2 only come in three shades or variants.
After the "Great War" Geckos began to change and mutate due to the surrounding Radiation. They grew taller and wider, and became giant, similar to the transformation of the Giant Ants. 200 years after the bombs dropped the inhabitants of the Wasteland still hunt Geckos for their skins, and the player in Fallout 2 can even skin a gecko with the Gecko Skinner perk.
There is no creature in the Fallout Games just named "Gecko", and there is a town named Gecko.
 Little GeckoMain Article: Little Gecko
Little Geckos are the most common type of gecko and can easily be dispatched. They are smaller than their counterparts. They have a pale color.
 Fire GeckoMain Article: Fire Gecko
Fire Geckos are a more mutated form of gecko. Like their name suggests they can breath fire because of toxins in their gut and stomach. Their skin is burnt and black and they cannot be skinned because of this. They are fairly hard to kill and it doesn't share the same appearance as the other sub-species.
 Golden GeckoMain Article: Golden Gecko
Golden Geckos are geckos that are found near high amounts of radiation and have a yellowish-gold ting to their skin.