[RPG] find a “monster level” for monsters in the AD&D 2nd edition manuals

The concept of "monster Level" is introduced in the Dungeon Master's Guide, Appendix C, page 174. The levels for the creatures in the Monster Manual are in the tables on the following pages of that appendix. The listings in the Monster Manual II include it right in the monster listing for convenience, because the book wasn't published when the Appendix C tables were compiled. As the Monster Manual II says in How To Use This Book (p. 6; emphasis mine):

LEVEL and EXPERIENCE POINT VALUE are determined by the method indicated in THE DUNGEON MASTER'S GUIDE. If a type of monster has varying hit dice and/or experience levels, values for each are shown.

Monster Level is a rough indicator of relative challenge, but is far from an exact science. Monster level is a direct function of the XP value of the creature, as calculated from the table on page 174 of the DMG. Monster level is roughly equivalent to the level of the dungeon on which they will be commonly found, though they can be found lower or higher, in greater or lesser numbers encountered.

Created as a rough guide for random encounters, it should be taken only as a suggestion, and as a means of sorting creatures into any random encounter tables you might create yourself. It was never intended to be used as a fixed measure of difficulty, and if you try to use it that way (like CR in WotC editions of D&D) you'll find your games become more unpredictable instead of more predictable.

DMGR5 Creative Campaigning (1993)

Creative Campaigning has stats for Fionn MacCumhail as an 18th level Warrior/8th level Druid. It used a similar style of book cover and interior design to Celts and came out around the same time.