Its red claws and the tips of its tail are melting from high internal temperatures that are painful to Houndoom itself.
– Pokémon Ultra Sun Pokédex
Mega Houndoom is still Dark/Fire type. It gets the ability Solar Power, which raises its Sp Attack by 1.5 in the sun, in exchange for losing 1/8 of its HP at the end of the turn.
Stat-wise, it gets a nice boost in Sp Atk and Defense, as well as minor boosts in Speed and Sp Def