Nyxborn Unicorn
From Modern Horizons 3 set
Nyxborn Unicorn
Enchantment Creature — Unicorn
2/2
Bestow
(If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached.)
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
Enchanted creature gets +2/+2 and has mentor.