The title kind of says it. There's a good decent amount of Display IDs that don't work with NPC Forge.
They give off the error: "Cannot find this NPC. If you made it in NPC forge try recreating as the previous Display ID or other data is probably incorrect."
From what I've tested, it only seems to be all the mounts or some. Which kind of sucks. But they don't seem to affect the class mounts from what I know of so far.
Are the IDs that I could get a hold of. I should've saved the others which would've been like 8-10 more but I can't exactly remember them so... Maybe another time