Gargantua, Tentacle, and the Vortigaunt all take the cake.
Headcrabs, Vortigaunts, Snarks ^_______^ cutieeee :3
I’m really bad at picking just one favourite of anything, but I’d probably say houndeyes. I’d want to try to raise one as a pet.
I also like the vorts, the Tentacle, and the Antlions (if we’re counting them; I imagine they lived on Xen, since the vortigaunts once used them as livestock). I loved working with the Antlions in Nova Prospket; I got so attached to them that when I got startled and accidentally shot one, I reloaded my save.
I like the plants in my real life, and i love to have a xen light
The Boid creature definitely.
I like it because it looks like a stingray.
And i like them as well.
^^
Gargantua!! because its really crazy xen creature xD
I’d definitely have to say the bullsquids, because they seemed so much like a real animal (acid-spitting nonwithstanding) and having them prey upon the headcrabs really drove home that these creatures are from an actual ecology and not just one big happy family of inexplicable evil monsters.
hahahaha bullsquids, i don`t like it… bullsquids are cthulhu childs
I’ll go with the Vortigaunts as well. They have always been my favorites, no matter if allies or enemies.
Head-Humpers. More iconic than vorts, in my opinion.
Tentacles are mysterious for me… I´ve always tried to imagine the rest of the organism (I know there were some burned parts of the body in the original HL at the end of the Blast Pit level, but that wasn´t enought) … plus their voice is frightening in some way (especially when they´re “screaming” in pain during the “barbecue”)
Vortigaunts, they remind me of humans.
A Houndeye? pfft. Panther Eye is way better.
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Mr. Friendly anyone?? 
Snarks are just cute.
Just… look at the animations!
youtube.com/watch?v=t4qywJaEv5s
The rare animations unfortunately don’t have sounds, so I just added my own…