Before the Gonarch became an Error model, I actually do spawn a Gonarch. Complete with everything, but particles, sounds and textures. It can charge, spit acids like Bullsquids, spit a high-damage blob. Instead of spawning a single Baby Headcrab, it spawns multiple Baby Headcrabs (no textures and sounds too).
Then there’s the Xentacle with a complete entity, but it seems buggy. Hitting it with explosives will send it moving SO FAST AWAY!! It functions like normal Tentacles from Half-Life 1 albeit you need to avoid them instead of killing it. Shooting it will make them come down to the pit, and ascend for a second later like in the original. They do not hit targets with beak; possibly that their code is still incomplete. They only sing.
You can spawn Fast Headcrabs and Poison Headcrabs from Half-Life 2! However, they do not have any sounds.
Then on the skybox game files, there are 6 Xen skies I saw. Xen_Sky_3 will most likely be used during the Resonance Cascade (when you get to see Bullsquids for the first time). Xen_Sky_4 has two suns (equally the same as the skybox, alien1 from Half-Life). Xen_Sky_12 and Xen_Sky_5 have a black sky as the background with green nebulae. 12 seems to be “away” from Xen, and it seems Xen itself is below on that swirly hole.
