To add to the above, Pathfinder T Rex (18 HD) has Bleeding Critical, Critical Focus, Diehard, Endurance, Improved Critical (bite), Improved Initiative, Iron Will, Run, Skill Focus (Perception)
Not sure about Run, Endurance/Diehard and Skill Focus (does it really need the +37 PER roll?) being better choices than Toughness (much better in Pathfinder than 3e) or save enhancers, but at least it got something.
Realistically, it would not play any different if we gave T Rex a racial bonus to Will Saves, Initiative and Perception, and wrote the rest up as monster abilities, but a lot of the 3e - PF model was about using same/similar rules where practical. Adding feats also means we get a sense of what we should add if we make this an especially tough, 24 HD T Rex - he also gets some more feats out of the deal, be they save enhancers, Toughness for more hp or more Critical feats, depending on the direction we want to advance him in. A much more defined "monster progression" model.
Mainly allows us to criticize published creatures who don't perfectly follow those rules, but que sera.
Anyway, yeah, monsters get feats. Orcs and goblins always (start of 3e, anyway) got feats as they advanced, I think, as they were normally advanced by gaining class levels. They are also playable races, add-ons in 3, 3.5 and PF IIRC.