This is probably old news to everyone in the world except me, but not all the monsters are evil?
I saw the "Bribe" and "Surrender" options of course, but those seem really bad, so I never use them, but then my alignment started slipping!

I looked it up, but mostly just got the advice "don't worry about it, it doesn't matter." which is fine, I guess, but if there is an alignment system in place I would like to play y good characters as good!
(I also got griped at by a god because my good leader was currently neutral, and he smacked be to the bottom of a dungeon into a fight with 6 dragons... really bad news for a level 6 party.)

So I started being more careful, I suck it up and bribe Pegasus fights, or try not to fight them when stuck in combat with them and they seem to run away.

My question is: What other monsters are good? I am thinking Unicorns and Clerics? Any others?

I don't think there's any way to know short of hex editing.
But alignments can always be restored.

It's a pity there's no in-game way of knowing, since it could have encouraged more role playing. But then all random encounters throw every kind of monster together, so it's kind of a moot point anyway.

I even had alignment shift on fighting Sprites right in Sorpigal (but not always, only sometimes, so there is some random chance involved). Pegasus is another such monster as mentioned. In the beginning of the game I solved this by trying to bribe the monsters with 0 gold, they refused my proposal, fight started and there was no alignment shift. Later in game changing alignment back was so easy that this became irelevant.