I gave up on trying to make my compost mounds drop mound tubers on butchering; instead they straight up create the veggies roughly once per season. Since the mounds are not domestic creatures and they cannot breed, the best use of dropped tubers is to brew them and plant the seeds.
Butchering a compost mound now yields a poor quality green tourmaline figurine that represents the creature's crystalline golem core.
I've also realized I shouldn't have made these lads out of actual soil, because they were a little too hardcore, with soil being stone and all that. Not only was it impossible to hit the core, but dealing any kind of damage was way too difficult. Now they are made of a custom material that's easy to pierce without being too flimsy.
Edit:
Now that I think about it, my strandcats may also be a bit unfair, being killable only by bisection. I'll give them a central node made of soft fungus in their upper bodies, and in return I'll make their heads non-essential.
Another edit:
Finally got my stuff together and started making those T'lethian pets. The first is the deep one - a chimaera made from alien and human embryos. It can discharge electricity, causing pain, blisters and sometimes paralysis to its enemies.
During the first test an unarmed human managed to defeat a deep one, which shouldn't happen considering electricity, size, and aqua-plastics body armor that should give the aliens the upper hand.
During the second test I took control of the deep one, and immediately went for a discharge. I dropped to the ground paralyzed, and my enemy stomped me to death. I can imagine things like that being detrimental to aquatoid invasion, so I made all TFTD creatures (currently two) immune to this syndrome.
I've also adjusted the T'lethians themselves a bit - tasoths and lobster men no longer have metal scales and shells; instead I went for material force multiplier. An armed dwarf of moderate skill can take down unarmed tasoths and lobster men, not to mention aquatoids and gillmen. I'm not sure how the things will turn out once the aliens come with aqua-plastics vibro blades and sonic-blasta rifles.