if you use ethereal mutagen on a companion that flies alongside you, the transformed pet will still fly, resulting in a flying mutagen. :D
i've heard of this happening before, but i never experienced it first hand. yes, that is a flying mutagen pet. originally it was a celestial dragon, but i was alt tabbed when someone used some of the ethereal mutagen on it. pretty awesome if you ask me! i've always wanted a little monster such as this haha.
it makes me a little sad, though. the pet consumable is only purchasable from an ethereal soul-trader (only your own, not someone else's), and unfortunately this pet is from an older TCG loot card and therefore quite pricey on ebay. i may never get my hands on one, so i guess i'll have to enjoy my temporary mutated companion while i can.