Guest monsters are monsters appearing in Act V of Diablo II alongside normal monsters, starting on Nightmare and Hell difficulty. They are based on normal monsters of all acts, and appear in a wide variety of placements in all areas of Act V.
Sometimes guest monsters are just like the monster they are based on, with slight statistical and level changes. But most of the time they have different resistances and immunities, attacks, and skills. They use names and color palettes of normal monster varieties, sometimes completely shuffled like the guest Infidel which has the look of normal Marauders. They can even appear with otherwise-unused assets, like the guest Razor Spine which uses color palette of the unused Bush Barb, or an unused variant name like Siren.
There are 39 guest Demons, 39 Animals, and 36 Undead, in total: 114 guest monsters. (Pandemonium not counted.)
The Pandemonium Event also has its own guest monsters, used as normal monsters of event areas. They work like other guests, but of course are much stronger.