Donkey

"Yes, what is it? Drop your donkey in the trash and don't even tell us what we want." -Unnamed UNGC Soldier

Donkeys are small, purple creatures that are rarely seen yet highly prized by the Unique Natural Gas Company. Only five are ever seen in the entire Mesa Black Universe: one that is hidden in a vent in the Liverpool Complex seen by General Adrian Physical Shephardt, one hidden in a box in a locker in PHYSCIA: all the forms of the Blue, and a hidden lair in the Lost World containing three of them. Additionally, constructing them appears to be a requirement for completing Boot Ice Cream, as Dwight S. Drill Master I. Ben tells Shephardt to "build a Donkey" while he's being shot at, and one soldier mentions that "some people are angry when they have to save [his] donkey."

It is possible that the donkeys mentioned by the UNGC refer to the real world species of the same name instead of the small toad-like aliens seen in the games, though given their pacifistic nature, their small size (and thus, easier-to-construct frame), the fact that they make actual appearances in game, and the fact that this happens to be the name that Google Translate spewed out for these creatures, it is very likely that these are the "donkeys" that the UNGC talks about in Half-Life: Defense.