Horn of the Unicorn is a Equip Spell Card.
Spell Cards are usually activated from the hand on your turn unless their type gives another timing window.
On the current TCG and OCG Forbidden & Limited lists tracked here, Horn of the Unicorn is Unlimited.
YGOPRODeck lists Horn of the Unicorn for these formats: GOAT, OCG GOAT, Duel Links, Common Charity, Edison, TCG, OCG, Master Duel.
A listed print of Horn of the Unicorn appears in Battle Pack 2: War of the Giants (BP02-EN134) at Common rarity.
Among markets shown on this page, the lowest listed snapshot for Horn of the Unicorn is about $0.06 on Cardmarket (prices change frequently).
