Majestic Auricorn

The life trigger keys off the word "mutates," not off the creature entering, and that distinction is the whole point of the design. Every time you stack another mutation onto a pile that already includes this Unicorn, the trigger fires again: four life per merge, repeatable across a game as you keep feeding bodies onto the same growing creature. A 4/4 with vigilance for five mana is a plain rate on its own; the card is built to be the base you keep building upward from, not a one-and-done body. Mutate lets a permanent behave like an Aura and a creature at once, with the spell going over or under a non-Human creature you own and the merged pile becoming whichever card ends up on top plus every ability underneath. This Unicorn is white's stabilizer for that pile, its answer to what keeps a mutate deck alive while it assembles, because building tall means committing several cards to a single body that one removal spell can blow up for a devastating multi-card swing. The life buffer offsets that fragility, and the vigilance matters more than it usually does on a defensive card, because the intended body is one you want attacking and blocking in the same turn. It asks you to treat each merge as an incremental gain, four life at a time, while the pile grows taller and more valuable and more exposed.