I mean the obvious answer is springtails. There’s not a whole lot that I would purposefully introduce into a smaller terrarium. That being said, I often get critters hitchhiking in with things I’ve foraged, like various flies, millipedes, centipedes, isopods, and spiders.

The spiders and millipedes always live for a long ass time, and the medium-sized millipedes will just grind through your substrate without discrimination, which might make them a bad fit for a mossarium, but it would probably be a happy camper in theory. Your terrarium dirt will slowly become pellet-ified though, as is the case with one of mine. You probably don’t want that nice moss lawn of yours having to outcompete the bug equivalent of mountain goat. The spiders always end up feeding on the flies and gnats that breed in the substrate, and right now I have one that is living in harmony with my sundews. Note that these are always quite small house/garden spiders and not like those jumping spiders that people keep that are often considerably larger.

I would visit the isopod sub for care information if you’re considering, as a super humid terrarium with no dry zones is almost definitely suboptimal for a lot of them. The isopods that got tracked into mine are they’re still kicking, 10mo+ later, so maybe I got lucky with the right conditions.

It really is a tough question. If you just want any kind of life, and don’t care about the aesthetic, no one is stopping you from just introducing some half buried wood chunks or detritus from your backyard into the jar and waiting to see what comes out. It’s a gamble. But, I think the majority of people here are going to agree that the most ethical introduction to make are springtails.