Snakemen were quite a common sight on Earth after a few months of fighting during the First Alien War. After the repeated terror attacks by Sectoids and Floaters, the Snakemen came out of nowhere and surprised us all.
Fortunately, by the time they arrived we had acquired far superior weaponry (in the form of Heavy Plasma rifles) and the threat posed by this race was reduced dramatically.
Snakemen are also frequent terror agents, and are accompanied at terror sites by the fearsome Chryssalid. This association means that X-COM squads attending Snakemen terror sites should be well prepared.
This race developed in an extremely hostile environment. They are extremely tough and can resist extreme temperature variations.
Their mobility depends on a snake-like giant "foot" which protects all the vital organs.
Their objectives appear to be purely predatory and they appear to be under the command of some other Intelligence which directs their military-style incursions on earth.
The skin is extremely tough and heat resistant.
The cardio-vascular system is part of the muscular system which uses the hydraulic principle to create motion. The only true muscle is the "heart".
The reproductive system appears to be very efficient. Reproduction is asexual, with each snakeman carrying up to fifty eggs inside its body at any one time.
Left to its own devices this species would be a severe threat to life on earth.