Mushroom is mutated Umbreon. He was an eevee with a trainer at first, evolved then was released. He wandered a dark woods for a while and stumbled across a leaf stone after he evolved and touched it, mutating him. He didn't turn into a Leafeon though, because he was already an Umbreon it just have him some differnet colours and powers.

When Mushroom was in the forest he found Autumn (see links, I hope the lore is alr with u wolx), who had also stumbled across another stone after evolving into an Umbreon. She already had friends as well; Firefly and Thea. (hope lore is ok with you cupcake and delta). Now they're all just frens living in the forest!