Abomination

 are the result of alien experiments on abducted humans. They are found on the board of Mothership Zeta. They appear to be a hybrid of a human and an alien, and they are hostile to anything that is not one of them, including aliens.

Loot
When they are killed their body can contain normal alien items like Alien Power Modules or Alien Epoxy but sometimes human items can be found like Pre-war Money, Harmonicas, Human Flesh, and Pre-War Books. (Books Confirmed 360)

They also emit "growls" that are similar to a Feral Ghoul's, further suggesting they're alien/human hybrids.

Appearances
Abominations appear in the Mothership Zeta add-on for Fallout 3.