If you picked Squirtle as a starter Pokemon, then when you arrive at Sea Cottage, you will be offered by Bill to fusion your Wartortle and his Eevee to make a Warvee. At level 36, your Warvee will evolve into a Blastoreon.

If your rival has a Warvee, your first encounter with their Blastoreon will be when you meet in Pallet Town.