Pages 10 and 11 of The Underworld & Wilderness Adventures (OD&D book three) contain tables for the determination of wandering monsters in the dungeon, one for each dungeon level, down to level six. These tables have eight to twelve entries each. In addition to containing information about implied setting, this collection of monsters functions in a certain way with other rules elsewhere in the game. While OD&D is robust enough to work just fine if other monsters are used, these linkages, and the roles that these monsters serve, are still interesting to consider.
Special monster attacks and defenses interact with equipment and character abilities. Monster defenses require special weapons to overcome. Silver weapons are required to damage lycanthropes, magic weapons are needed to combat gargoyles and many kinds of undead, acid or fire is needed to fully destroy trolls. Monster attacks can be resisted by certain defenses or cured by particular resources. The antidote to poison is the fourth-level cleric spell neutralize poison, petrification can be fixed by the sixth-level magic-user spell stone to flesh, elves are useful for resisting ghoul paralysis, mummy disease (which impairs recovery) can be handled with cure disease, basilisk gaze attacks can be reflected with mirrors, scrolls of protection are useful against entire monster groupings. And so forth.
Another pattern to note is that most levels draw monsters from the same set of categories, though not all levels have examples of monsters from every category. For example, magic-users only show up on levels two and deeper, and giant animals only appear on the first four levels. All the monsters in a particular category may not just be palette-swapped, but they do tend to broadly share qualities such as types of attack and vulnerabilities. The most common wandering monster categories seem to be: fighters, magic-users, undead, humanoids, giant animals, dragon types (though only on the deepest two levels). This is important because these categories communicate threat information to players that can be used profitably against more powerful variants of the same monster type.
The content triumvirate of monsters, equipment, and spells work together as a set of interconnected, opposing relationships. Monsters have strengths and weaknesses, which can be defended against, or exploited by, the tools available to players, which include those aforementioned categories. Replacing these elements with new, custom content is a common method of constructing a unique and surprising campaign setting. By no means do I wish to suggest that this is inadvisable. It may even be necessary to engage or challenge experienced players. However, it is probably worth considering these game mechanical relationships and making sure that similar dynamics exist within new collections of monsters as well, rather than making every creature entirely unique and unpredictable.
See also, regarding interactions between game constructs: