I've spent quite a while creating my own creatures and civs.
I created an orc civ with orcs as a separate race from goblins, made some more civilized, friendly by default, kobolds called Cobalds that live in retreats in the mountains, copied all the semi-megabeasts and made an evil civ that chooses between them, and made a human-sized copy of tigermen living in cities that steal items from surrounding civs, making them enemies by default. There were others that I essentially scrapped.
I made a mushroom-man mod, with a few different species each offering something unique and interesting, including a milkable common-domestic species.
I began making some new underground creatures, a cave whale resembling early concepts of cloverfield for one, and a real-world salamander called a mudpuppy for another, then some magma crab variants that need their materials updated so their limbs don't fall off anymore, but haven't added any in a long time.
I then made creatures resembling those on an alien planet show, that basically spend their entire lives in the air, like a sky whale and hug three-eyed birds, a floating jelly-fish like thing with draping tentacles, and a tree-like creature that I want to make slowly digest things alive. I don't use them much because they have a tendency to take over the world, somehow.
I duplicated cave spiders a few times and made them into new species for outdoor use, then made three new variants of GCS that don't shoot webs and can appear outdoors.
I also implemented some dwarven swine someone posted somewhere else, making the females war beasts and leaving males small and weak.
Recently I added a few new species of draconic creatures; wyverns, a basilisk whose venom won't work right (only some of the syndromes will take effect), even more massive emperor dragons, the only new megabeast, and a non-megabeast version of normal dragons. The mountain wyvern, basilisk, and feral dragon simply don't want to appear in worlds you generate, even when they are reconstructed based off one of the functioning creatures, but none of the creatures generate errors when initializing or fail to show up and live just fine in the arena. They all have a new skin type, dragon scale, having the stats of copper and providing dragon scale for armor.
And I've been making custom reactions that create and 'read' books to train dwarves in a skill. It was all going peachy until I realized it would create thousands of individual books from any single roll of cloth. Despite many attempts I can't fix this, so I'm considering making them from leather or stone or something instead just so it works right now.
I really feel like reimplementing some gemstones I made in 40D for use as reagents in some reactions, as well as adding in 'meteorite' stones found in various layers as small clusters or singular pieces, then making them smeltable into various specialized alloys with iron for use with the gems.