Rainbow Dash, sleeping on Cumulus, her Altaria. Although clouds are nice to sleep on, Cumulus has the added perk of being very warm. Warm, and fluffy=Best sleep ever.
Rainbow Dash probably found Cumulus as a swablu that got separated from her flock, and was too weak to keep up. Hiding in clouds, Rainbow ran into her during her training, and decided to train her into the best Swablu/Altaria for the next migration. But, she got so bonded with her, as did Cumulus did with her, that instead, they stayed together.