I play a mix of AD&D 2ed and 1ed and I need to know if monsters are supposed to have THAC0. I can't find the monster THAC0 chart anywhere. I only have the 1978 edition of the Monster Manual. My friend showed me how to start a dungeon but he forgot to tell me about THAC0.
Yes, Second Edition monsters have THAC0. The Second Edition monster books were first the Monstrous Compendiums (looseleaf binders) and then later the Monstrous Manuals (hardbound).
AD&D Second Edition came out in 1989. You appear to be looking at First Edition AD&D monster books (Monster Manual, Monster Manual II, Fiend Folio). These do not have THAC0. It is reasonably easy to give these THAC0s, of course - you take what they need to hit armor class 0 off the 1e DMG table on p.75 and that's the base THAC0. 1-1=20, 1=19 1+=18,2-3+=16,4-5+=15,6-7+=13, etc. Modify by strength.