Vortigaunt

Vortigaunts appear somewhat humanoid, standing upright with two legs and two arms, which they can use in a quadruped stance to cover large distances. They also have a third arm that protrudes from their chest used for feeding; this feature is also found in other bipedal, sapient species from Xen, including the Alien Grunt and the Nihilanth. This similarity, along with other shared features such as red eyes and digitigrade legs, suggests a common ancestry with other intelligent Xen life forms. Half-Life: Alyx's Vortigaunts use their green energy through this third arm.