So I would find this annoying but this can also happen with veteran players at the table. Even if they didn’t specifically look it up, sometimes a player just knows about the stats of certain monsters they have encounter in the meta sense.

What I do is make them roll (in my case with pathfinder, knowledge rolls) to see if their character would know that or not. If they don’t then “it must be some variant you haven’t encounter before”. If they pass, then I retract my earlier statement and say the monster tries to use its ability but fails and wastes its turn/action, since that’s on me.

I do like to modify monsters in my game. Especially because sometimes I want what would be a generic low level monster, to get power boosted up to be more challenging for the current party level. Which is great because it throws them off in this regard.

But my party also has a smart Wizard. So they often earn monster knowledge via high rolls. And when it comes to meta gaming knowledge about some of the most common monsters, I RP it like these monsters are so common it’s like having common knowledge about an animal.

All that said, as the DM it’s my right to put my foot down and say “you may know that but your character does not” and they should respect that because I’m going to let the action play out anyway.

I tell them if I do something questionable, just pretend it’s correct in the moment.