This seems weird. I’ve seen it hurt Orion’s Charmeleon and some old lady trainer’s Bellossom, but that’s it.
I think Orion’s Charmeleon has Solar Power as it’s ability, that’s why it gets damaged by sun
2 Abilities, Solar Power and Dry Skin have that drawback in exchange for some benefits.
Solar Power: During harsh sunlight, Solar Power increases the Pokémon’s Special Attack by 1.5×, but the Pokémon loses 1/8 of its maximum HP at the end of each turn.
Dry Skin: During harsh sunlight, a Pokémon with Dry Skin will lose ⅛ of its maximum HP at the end of each turn. During rain, a Pokémon with Dry Skin will regain ⅛ of its maximum HP at the end of each turn. Fire-type attacks do 25% more damage to a Pokémon with Dry Skin. When a Pokémon with Dry Skin is hit by a Water-type move, its HP is restored by one quarter of its maximum HP, and the move will have no effect on that Pokémon (if a Pokémon with this Ability already has full HP, the Water-type move will do nothing).
That’s wacky. You’d think there’d be a wiki entry for these that google would turn up when I plug this same question.
Well, for official stuff check bulbapedia, not this wiki, and try checking the Sun article.