Chryssalid

Nasty. Toothy. Very very fast. Able to implant an egg (that turns you into a Zombie) with little regard for body armor. The ability of single Chryssalids to multiply and wipe out entire squads makes them more feared than even the tougher Ethereals.

Note though that their lack of ranged attack or flying ability makes them easy meat once you have flying armor.

Weak against explosives.

A HORRIBLE way to meet one is shooting a relatively harmless zombie with reaction fire as it shambles in for melee... then a Chryssalid pops out and SPRINTS at you, turning your squad into cheese.