I remember from many years ago, I had a pegataur ranger in a DnD campaign. I can't remember which version of D&D it was from, or which book it was from. Any ideas?
I do remember that the book had a brown cover.
There are no specific rules for playing pegataur rangers in any edition of D&D prior to 3rd edition. (In 3rd edition the pegataur is a playable race simply by virtue of generic monstrous characters rules, and there are no special pegataur/ranger rules.) However, the "brown cover" is a dead giveaway for an AD&D 2nd edition campaign using one of the "Complete" handbooks.
Despite there being no specific rules for pegataur rangers, I think I've figured out what you're remembering. What your DM likely did was use the loose recommendations in the Complete Book of Humanoids (pp. 15-16) to create a playable race from the pegataur monster entry in the Monstrous Compendium, Mystara Appendix (p. 86), and then used the "Wilderness Protector" fighter kit in the Complete Book of Humanoids (p. 72) to create a ranger-equivalent fighter-based class for your character. This kind of ad hoc character-creation rules based on loose recommendations and GM permission was common in AD&D 2nd edition, so it's no stretch at all to figure that this is how your past pegataur PC came to be.
Prior to AD&D, the only appearance of pegataurs a player-character race is in Top Ballista (1989), which describes a flying gnomish city as a campaign setting/expansion in the Mystara world. However, since this is a Basic Dungeons & Dragons book, PC pegataurs advance with a race-as-class paradigm and aren't able to be a separate class like ranger as well.
The most likely answer is The Complete Book of Humanoids for AD&D's Second Edition. It's brown, and has the rules for centaur characters, which would probably include variants like the pegataur.
I'm still continuing my research, but what I'm finding is that pegataurs appeared in the following AD&D/2e books...
But that's just the creature. I imagine that your ranger just focused on them as a monster and there's not a random specifically dedicated class.