Cheapest priced copy (Commander Masters): $0.49 at Card Kingdom, near mint, their price list as of .
Loyal Unicorn 
Creature — Unicorn · 3/4 · Uncommon · Commander Masters · #40
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.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
3 printings · $0.49 to $1.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2018 | 4 | Uncommon | $0.59 | — |
| Commander Masters | 40 | Uncommon | $0.49 | $1.49 |
| The List | C18-4 | Uncommon | $0.79 | — |
5 rulings from Wizards of the Coast
- 13 July 2018Once Loyal Unicorn’s lieutenant ability has resolved while you control your commander, its effects persist this turn even if you lose control of your commander or Loyal Unicorn.
- 13 July 2018Loyal Unicorn’s effect will prevent all combat damage that would be dealt to creatures you control, even if those creatures weren’t on the battlefield or weren’t creatures when the effect resolved.
- 13 July 2018If you don’t control your commander as the lieutenant ability resolves, you won’t get its effect.
- 13 July 2018The lieutenant effect happens only once each combat, even if you somehow control multiple commanders (perhaps because you have two commanders with a partner ability from the Battlebond™ set).
- 13 July 2018If you have multiple commanders, you need to control only one for the lieutenant effect to happen.