Snakeman Wiki entry
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.

Snakemen usually won't show up for at least two or three months into the game. As their name suggests, they are reptilian, resembling snakes with arms, slithering along on their lower bodies. They have noticeably better armor than floaters or sectoids, and are capable of absorbing significantly more fire than either. Their combat stats are at least comparable to Floaters, possibly slightly better. They seem a little more willing to use grenades than Sectoids or Floaters, too. Their key flaw, however, is low mobility. They are not able to move very quickly, slower even than Sectoids.

However, larger Snakeman missions involving Terror Units can be truly fearsome, with the inclusion of their devastating Chryssalid support units.

Snakemen take half damage from heat based attacks-
Blast Pistol, Blast Rifle, Heavy Blaster, Microblaster, Plasma Projector

Snakemen have a 1-3 on D6 chance of aborbing damage through their carapace similar to a Zoallan's Carapace - reducing damage by 2 points.

Snakeman Leaders count as being of every class - thus they receive one free reroll for each profession.

Snakeman Commanders are as skilled as Leaders, but also have 5 points of luck that can be used in the same manner as the players.