So what is a harpy exactly? It's certainly not an ooze. It's not a celestial, dragon, or construct. It could be a fiend or a fey theoretically though that's never been part of their lore. It's too intelligent to a beast, too Medium to be a giant. I suppose it could be an elemental but unlike Aarakocra, harpies have no traditional connection to the elemental planes. What about a manticore? More intelligent than a beast, with vaguely human facial features mixed with a hodge podge of animals. What about a Chimeara, which is literally a bunch of animals stuck together?
A catch-all category is kind of necessary because some monsters are just random mythological creatures that grew out of folklore and bear no nods at all towards taxonomy. And it replaces the 'magical beast' category from 3E and 4E so it's ultimately just a renamed category that's been in the game for 25+ years.