Some of you might be familiar with GURPS, which is definitely a backronym meaning Generic Universal RolePlaying System. Basically a general RPG ruleset to handle any era or genre.
So of course there are GURPS books to support Dinosaurs. In fact, they have three of them! They are Dinosaurs, Ice Age, and Lands Out of Time. All of them feel very 1980s, but then again most of GURPS does.
GURPS – Dinosaurs
The best of the three books, in my opinion. It’s also the longest at 130 pages, and is thick with “scientifically accurate” data of various dinosaurs and prehistoric creatures (as much so as a book from 1996 can be). The bestiary is organized by time period, so you have the Paleozoic era, Triassic, etc. There is a bit about cavemen as well, so it basically nullifies the GURPS – Ice Age book. If you were to pick up one of the PDFs this would be the one. The creatures and details inside would work well for someone running a campaign game of Dinosaur Cowboys since you could throw a lot more than the generic dinosaur types listed in the core rulebook.
GURPS – Ice Age
This book focuses more on prehistoric humanity, aka cavemen. There is a good bestiary section that has some non-dinosaur prehistoric enemies like Cave Lions, giant bears, mammoths, etc. The cover art is quaint and fun, and the interior is 66 pages long.
GURPS – Lands Out of Time
The newest of the sourcebook called Lands Out of Time is probably the closest to Dinosaur Cowboys as it focuses on human-dinosaur interaction. It’s only 43 pages though and a little light on art, and I think was mainly released to have some support for their later editions of GURPS.