Bullsquids were alien enemies from the first Half-Life, though they don't appear in Half-Life 2.|
Barney also appeared in the original Half-Life. A number of times, in fact. The Black Mesa facility was staffed by numerous security guards, all of whom looked and sounded alike, and all off whom delivered the same responses when you interacted with them. One of these responses was: "Catch me later, I'll buy you a beer!", and upon meeting up with Barney in HL2, he immediately reminds you of that offer.