Abberations, Undead, Planar entities, Dire creatures... D&D has all manner of categories for the entities that players may face in their travels. Each has expected traits and common abilities, even they do cover a wide range of options within those categories.
I'm thinking of doing something similar with The Fen, where I'm already up to about 40 different creatures that might be encountered, but these are generally categorised into
Blackblood - these creatures are infused with a parasitic black fluid that mutates and transforms them in unnatural ways. They take extra damage from weapons coated in a specific blend of fungus.
Fen - these are natural creatures of the fen, they may not be typical creatures on Earth (such as Fen Shamblers), but there's nothing particularly supernatural about them.
Shadow - these are empowered by the perpetual darkness enshrouding the Fen, such creatures would effectively live forever if they weren't killed by violent means. They are fearful of fire, and take extra damage from it.
Spectral - these creatures are undead spirit entities, echoes of their former selves who manipulate the world with ghostly powers and manifestations. They may only be disrupted by weapons of obsidian.
Swamp - like the "Fen" creatures, these are natural beings who live in the region.
There are a few other creatures in the list which don't fit into these five categories, but generally there are about six creatures fir each of those types, and when someone chooses a special enemy type, they'll go with one of those categories.
At the moment I'm populating the lists of creatures with various abilities, and the typical components they can be harvested for when they are slain. That's all part of the game, creatures are killed, harvested for parts that are combined with scavenged items, and then new weapons, armour and other items are created to improve potential survivability.