What are the Race X aliens, are they the Things on four legs, the squidshooter, and the other thing, like the shock gun, with the dude attached to it(well, its the other way around) or do you mean the Big blue thing at the end. What the heck was that by the way
Also, i think we can get to know all of the "remembered monsters" easier than we can name the forgotten ones. Headcrabs, zombies, barnacle, vorginant, and ichty(sort of). Ok, now we can say that Valve didn't want the others in because of issues(wether gameplay or they belong to gear box) or if you want to make the HL universe seem more real, the Xen creatures all died when you killed Ninilanth, because he countrolled them with his mind. The ememies from Op4 and blue shift all died when black mesa was nuked. There. Wasn't that simple?