Supplement books or make your own monsters. I love making a custom monster. I had really fun encounter with a custom ooze that could use engulf as a legendary action. Fun to have it chase the players around the map

Attempting to make a monster for literally every situation is how AD&D ended up with the Monstrous Compendium’s fifteen volumes and appendices, so in reality the Monster Manual is an exercise in restrain

I love the strange monsters from AD&D. Its fun to see how weird they got. I am a big fan of having lots to draw on for inspiration

Compare it to let’s say Egyptian or Greek mythology, same story.

Also DnD likes to add in monsters from different mythologies so there is that. Lots of inspirations to draw on

So I have been DMing a DnD campaign from level 3 to level 15 at this point. At this point my players have killed ~900 “monsters” at in around 40 different adventures. This doesn’t even count the numbers that were in an adventure they didn’t kill because they ran away, avoided or were NPCs with monster stats. They have killed at least 10 in all categories except plants, fey and celestials so having lots of categories helps as well.

So we need a lot of different options to keep everything fresh and interesting for the players and myself. Some of these are nuanced adventures with complex interactions and moral dilemmas while some are basic hack and slash. So I need a lot of different monsters to use that fit different flavors, use different mechanics and cover different difficulties. I have created some from scratch but it helps to have templates to start with. I won’t use everything in the monster manual but I have used a lot of it so far.

I think it helps to think of each adventure as its own little novel. So in one we are exploring the culture and cruelty of the Yuan-ti in a vaguely Mesoamerican inspired setting while the next they are exploring Bridgerton inspired high culture and dance that was infiltrated by both a rowdy fey and cosmic horror beyond the stars. It helps to have lots of inspirations to draw on since every adventure can be different.