JonnyP71
Explorer
Seeing a Peryton before, or even currently, is not a requirement for guessing that a creature you see is a Peryton.
Nor is it a requirement for having heard a story about a Pertyon that was not seriously wounded by most of the weapons on hand, but did suffer a significant wound from the one magical weapon on hand.
And really, a Peryton is a poor example of a character not being able to guess correctly what foul beast it is that they face, considering there aren't a whole slew of things that look similar to man-sized birds with elk heads to get the description confused with.
It's not at all meta-gaming for a player to have their character believe they are facing a Peryton because an elk-headed giant bird is attacking them. It's not even meta-gaming for a player to decide their character heard some folk tale about a Peryton unless a detail that hasn't been mentioned is that there is no such thing as Peryton on this world and this encounter is the first sighting of a new or alien species.
But it is meta-gaming to be upset that the player has had their character guess, and happens to actually be correct.
OK if they pass an Arcana check to recall said piece of Lore.
Fail the check and their character does not know the creature is a Peryton, let alone any of its abilities. All the DMs I know enforce this (including myself), and none of our players have any issue with adhering to it.
A great way to stop everyone dumping Int.
And I've never played in a group which enforces the Cleave rule.