Is Loyal Unicorn legal in Standard, Modern, and Commander?

Loyal Unicorn is legal in: Legacy, Vintage, Commander.

Loyal Unicorn

Q: What type of card is Loyal Unicorn?

Loyal Unicorn is a Creature — Unicorn. Its oracle text reads: "Vigilance Lieutenant — At the beginning of combat on your turn, if you control your commander, prevent all combat damage that would be dealt to creatures you control this turn. Other creatures you control gain vigilance until end of turn."

Creature — Unicorn

Vigilance

Lieutenant — At the beginning of combat on your turn, if you control your commander, prevent all combat damage that would be dealt to creatures you control this turn. Other creatures you control gain vigilance until end of turn.

Set Commander Masters

Rarity Uncommon

Mana Value 4

Artist Volkan Baǵa

Collector # 40

Scryfall Gatherer