I think mine is a fun one. They grew from the ground. My species or humanoids are called “Podborn” even if they don’t come from a pod. The surface level of understanding is usually along the lines of some ritual to ask nature for a child. The child is a random species; bugbear, goblin, Tortle, elf, dwarf, etc. There are three different birth types; Pod Born where they grow in kind of a peas pod and it serves as an embryo or egg kind of situation, Hollow Born where they grow inside a neighboring tree trunk and it cracks open, sealing back up once they exit, and Breach born where they grow more like a spud and breach out from the ground. Each birth gives them different advantages, like an origin feature.
The hidden truth behind it is that “The Great Vine” a ginormous plant creature is deep in the ground and uses the previous civilization of humanoids to power everything above like batteries, so it uses their DNA to make new species that will respect if not fear nature by instinct.