In the Desert Resort, you will find several statues by the entrance to the Relic Castle. When you come near the entrance, Professor Juniper will appear and will tell you about these Darmanitan. These ones are special in that they have a different ability to the normal Sheer Force ability and are awakaned by using a RageCandyBar on them. Instead, they have a new ability called Zen Mode, which, when Darmanitan's Hit Points get below 50% of maximum, will make Darmanitan change form into a new Fire/Psychic Pokémon
Also, they're at level 35, making them consistent with the game progress. On the other hand, Musharna and Volcarona which are obtainable only after the Elite 4, are at Levels 50 and 70 respectively (note that this isn't proof).
Further, this probably should be higher up, but I distinctly remember encountering them in my play-through (unless my game was glitched :P ), and catching one. So personal experience corroborates with the answer. Go get yourself a Darmanitan Z! :D