Tentaculat
Not even the depths of a Lovecraftian nightmare, nor the darkest aspects of the Cthulu Mythos, could create a monster as horrific as the Tentaculat. It is the hypothetical aquatic cousin of UFO Defense's Chryssalid, only tougher and possessing the power of flight, due to the fact that the Tentaculat move by swimming around at a breakneck pace. They are classified as a Terror Unit, yet they are often found guarding large Alien Subs and Alien Colonies, since they cannot participate on any terror attack on the surface due to being strictly aquatic. They are also found defending T'leth with incredible numbers.
It is unknown what sort of alien environment would create the Tentaculat, but judging from its biological features, it's safe to assume that it originated from an extremely deep ocean environment that exists in total darkness. Tentaculat appear to be giant tentacled brains with a sharp octopus like beak. They have no eyes, but they have a natural heat and motion sensor, so they are not blind. They are incredibly fast and agile, and their tough yet misleadingly soft-looking skin can take almost as much punishment as a Lobster Man.
Their most terrifying feature is their primary attack. Like the Chryssalid, they are able to turn Aquanauts into mindless zombies, that also double as walking egg-sacs for Tentaculat embryos.
The impregnation methods used by the Tentaculat are similar to the Chryssalid in some ways. After choosing a target, they make a beeline for its intended victim at speeds rivaling the Chryssalid's running speed. Once it's right next to the host, it stuns him using its tentacles, then delivers a bite with its metal-shearing beak. This bite does two things. One, it injects a poison that causes the body to bloat and go into rigor mortis, increasing physical attack strength while inhibiting independent thought and self-awareness to the point of mindlessness. Second, it carefully implants an embryo within the body of the now zombified host. The embryo feeds upon the host's bodily fluids and tissues, growing at a rapid pace. Killing the host body causes the baby to instantly consume whatever nutrients it can get, absorb the said nutrients to fuel one final growth spurt and then finally burst forth from the withered husk of the host's body. This attack is not only frightening, but if the player is not careful, the entire squad of Aquanauts could be overwhelmed by the Tentaculats bursting out from the skin of their not so lucky comrades. However, there is a way to prevent a Tentaculat from being born. Using gas cannons or torpedoes loaded with Phosphor rounds, which burst into flames upon impact like Incendiary rounds, on a zombie will kill not only the unfortunate host but the embryo within it.