dave2008
Legend
That is on you, not the DM.If I was playing in a game, and the DM told me that the giant spider we fought was just a re-skinned dragon, then I'd feel kind of cheated and disappointed that they didn't care enough about the game to actually put in the work.
If I re-skin a monster into a new monster, then it becomes that new monster. There is nothing to reveal. If a player asked, where did that monster come from. I would tell them - no big deal (though I have never had that happen). Why would some ask where a monster came from? That is oddly meta. I mean, usually the answer is the "Monster Manual," why would a player want to know that. I guess I don't usually give players info the characters don't have. Kinda of a strange question.I mean, if there was nothing wrong with it, there would be no reason to even hide it from the players. You would be able to tell them right off the bat, and they wouldn't think anything of it.
Anyway, as someone else pointed out, it (re-skinning) is literally the first advice in the DMG on creating monsters.