You may be thinking of the Cetalid...the gaint kidney that spits acid. A Chrysalid is the black crablike terror unit that comes with Snakemen and can impregnate your entire squad Xenomorph style.
They have uber stats and a hand to hand attack that can take you in one hit and turn you into a zombie. I believe they have around 120 time units, 90 reactions, and around 100 health. Quite a mean customer if you are on the ground with them. Heavy plasmas are almost a must on higher difficulties.