The thing with animal ponies is odd. If the subterranean ones are still appearing, it might be that [DOES_NOT_EXIST] isn't working for wild subterranean creatures, which would make sense of a mention of serpent men I saw in the preferences of one pony. Or, if you're using the default entity_default.txt rather than the MLF one, they might be appear as part of underground civs, because entity creatures are known to disregard the DNE tag. The MLF version replaces them with animal ponies.
As for slug, snail and leech men appearing, that's a derp, I should have included an overwrite for creature_other.txt.