Lion is one of Princess Luna's royal guards was in Canterlot during the Pink cloud strike. The strike turned him and many other ponies into Canerlot Ghouls. Mouse was once a dragon who guarded the gold under Canterlot castle. The pink cloud merged her with her horde and caused her to breath pinkcloud instead of smoke. Thanks toa Magic spell created by a Crazed Alicorn, she was turned into a mouse.
Lionheart and Mouse formed a formidable duo, Lionhearts flight and combat experience allowed them to take down Enclave attackers and fly around the wasteland quickly. Mouse's ability to breath pink cloud is also a potent weapon in their arsenal.