Flutter Mane actually makes a lot of sense
Misdreavus’ source of food and nutrients is the vitality it gets from scaring people by playing pranks or making spooky noises. It doesn’t deal well with folks who aren’t easily frightened.
…And then we have Flutter Mane who is a supposed ancient version of Misdreavus who is a lot bigger and beastlike
Raifort the History Teacher: “People back then were far more likely to believe in mysterious legends, magic, and beings beyond human comprehension.”
The lack of human advancements in the far past means that fear factor of the unknown was a lot more intense and imaginative. This may be where Flutter Mane gets its Fairy Type from as people were more inclined to believe in made up things like Fairies. But as civilization advanced people nowadays lack a certain fear factor.
Also I like to think of Flutter Mane as a prankster’s fall from grace. Once you surprise your audience it’ll get a good reaction out of them but keep doing the usual tricks and they’ll get old quick. Ancient even.