Ravenous Bugblatter Beast of Traal
He did not have pre-established numbers. He didn't have AC, attack bonus, damage, HP, special abilities, or anything. He made them up on the fly. I still consider it fudging, as he was determining our roll results based on whim in the moment "Oh he rolled an 18...yeah I guess that should hit". It makes no difference if there was no pre-existing AC determined vs. he changed it at the last second. In the end, our rolls only mattered if he decided they did.
But earlier you said:
I'll just say that for me, Fudging is any time a DM changes an outcome based on their own desires, and not the pre-established mechanics of the game or encounter. Turning a crit into a normal hit because you don't want to kill a PC is the DM's desire, not the outcome of the dice. Increasing the HP of a monster that started at 100 to 200 because you aren't happy with your boss fight being so easy, is DM desire, not mechanics.
Coming up with an on the fly encounter is definitely not fudging. Like you say, reskinning, changing damage amounts and HP or AC is fine. Once the fight begins however, if you decide to change those stats mid-combat, that's definitely fudging.
Help me understand why this is fudging because your posts seem to say different things. Is it because he didn't have any firm ideas going into each encounter and made it up as the encounter was progressing?
Just thought of something - when you say "roll an 18", do you mean the result of the to-hit roll was 18, or the natural number showing on the die is 18? Because the latter would be ignoring the mechanics altogether and would annoy the snot out of me as well.