If I understand right, DF currently have codes that can be used (with a little changes like renaming, resizing et cetera) for procedurally generate and then spawn some of planned night creature types.
Raising of ghosts by necromancer. Why they are not in current release?
Evil savage beasts, like hexapod capybaras. Their bodies can be generated the same ways as FBs and experiments in current game. They just created by evil regions, not by histfig. Evil carps shall be like this, but aquatic.
Grendels, aquatic trolls. They can just live in water, like oceans, lakes, rivers. They not need boats now. If bogeymans was added with not all features, swimming trolls can be added too.
Cursed seafarers. The same, as aquatic trolls. They can be water version of intelligent undead, spawned by region (not histfig). Before boats, they shall just swim.
Constructed creatures. They can be done like experiments, but needs some RCP bodyparts for grafted weapons and extra butcher objects for using this weapons by adventurer.
Animated furniture. For firstly adding, game needs only new itneraction type, based on undead animation, but that can animate furniture, not corpses. And also bodies for each furniture type. This may be regional interaction like in current regions that reanimate corpses. When you embark in that, you will fight with your own chairs!
Animated trees. A I know, modder LargeSnail made them by DFhack, so they needs a little coding for first arrival.
Stalkers. They needs a lot of coding. But if trying to make them fast, they can be done as replacement of Murderous Ghost and with poperties from Intelligent Undead, they spawn as ghosts do, but healed and with real bodies, respawning after each death.