The very first legendaries were just some stronger than average birds. Except there's no legends for these guys in the games. The only time they were actually shown to be deserving of the "Legendary" title was in the second movie. And the other one was just some genetic experiment, thought that one at least has the excuse of being a clone of another Legendary/Mythical Pokemon.
As for why Kubfu is legendary:
https://swordshield.pokemon.com/en-us/expansionpass/pokemon/kubfu/>Wild Kubfu live in mountainous areas far away from the Galar region today, but it was once a species that called Galar its home. It’s noted in historical texts that this species used to head to uncharted lands with people as they set out to trade or explore. Records suggest that the Kubfu living in faraway mountains today may have descended from Kubfu that became naturalized to the distant lands they visited in their journeys.>historical texts 