RPGMemes

The only time that I've seen this monster introduced into a story is through the "story" that's built into Strixhaven. I use that word very fucking loosely because they leave an enormous amount of work to the DM. At any rate, that story is across several years in a magical Hogwarts-y school. In Year 2 or 3 they gain access to this big boggy area and have to go looking for shit. While spending time out there they can come across tracks and stuff of these monsters. I think there's something that also says a professor is supposed to mention something vague about these types of monsters.

In that case the expectation is that the players have just been paying attention to the game in general.

Otherwise yeah I agree in having a roll although only if the player asks for one. If they fire off the spell first without thinking and then inquire I would still allow the roll but the spell is cast. Only thing you'd get from that is advantage on the dex save.