I was checking my old AD&D books, but the original monk class seems to be very different from what is in BG2 and the class was removed completely in the ADD 2nd edition. What did BG2 base the monk character abilities on?

From third Edition D&D.

Yes, Third Edition. Bit still, the BG2 Monk is very different from the 3rd E Monk too. It's just an issue of balancing (which abilities would make the character too powerful, which ones wouldn't be powerful enough, which ones are just right) and implementation (what can be done in the Infinity Engine within the time limits for producing the game and what can't).