The aliens are a species that has developed an aggressive space faring civilisation. They stand over two meters tall clawed bipedal anthropoids with a nautral exoskeleton. They possess a smaller extra set of arms for precision manipulation which they can extend from their chest cavity the chest exoskeleton plates, reveling their more fragile interior.
They are known ot have come to earth in 1873 in Arizona to replenish their spaceship's fuel supply of gold and experiment on humans. They were planning on exterminating all life on the planet upon leaving, like they did on a planet they visited before but were thwarted by the local population with the help of a surviving member of the species they exterminated prior to coming to Earth.
As they dont seem to have plans of conquest, it is likely that they are on an eradication crusade of all species that could eventually pause a threat to their supremacy.
Source: Cowboys vers Aliens movie