Loxatl
The Loxatl are a minor xenos species. They're a species of mercenaries with no known homeworld, and they're also distilled awesome. Basically giant murder-geckos with underslung flechette weapons which they control with their minds. Loxatl hunt in groups, and while their vision they can taste you from kilometers away. Gross. They're also able to sense vibrations and breathe underwater, which is neat.
They're often used by Chaos cults and Tau as assassins because of their ridiculous speed and fine-tuned senses. Although apparently their one weakness is that they can be detected by the smell of rancid milk and crushed mint. Gross, again. This is assisted by their apparent resistance to las-weapons, although not enough to be immune to some good old-fashioned flashlight spam.