Swimmer

From Alien Species

In the Aliens Colonial Marines story published by Dark Horse Comics, a group of marines ends up on a colony planet known as 'Bracken's World', a primarily oceanic planet that grows large amounts of sea kelp. Having become stranded on one of the kelp beds following an APC breakdown, the group is attacked from the water by large Xenomorphs resembling Queen aliens from the top half, and a more whale-like structure below the waist with a lateral tail fin and several trailing tentacles.