Page 174 of the Scholar’s Guide (r44) makes a 1e mechanic more consistent in that all monsters are assigned a type. There are eight types after looking through the bestiary: automaton, beast, dragon, folk, ooze, spirit, troll, undead.
Most are self-explanatory. “Trolls” are feylike creatures from folklore, those spirits of the wilderness made flesh, and include most of the traditional monsters. There are a few misnomers, like “giant rats” were renamed to “troll rats” despite being beasts. This was probably done to avoid confusion with the giant type from Middarmark, but they should have been renamed to “dire rats” similar to “dire wolves”.
Some of these are unnecessary. Automaton and ooze could be combined into a sleepless ‘construct’ type that covers all magical creations. Spirit type currently only has two entries, goblins and ghosts. Both would better fit other types - the former are trolls, and other entries even describe them as such. The latter are obviously undead and should not be excepted from magics affecting undead. (dhb228)