Cheapest priced copy (Commander Masters): $0.35 at Card Kingdom, near mint, their price list as of .
Loyal Guardian 
Creature — Rhino · 4/4 · Uncommon · Commander 2018 · #31
Trample
Lieutenant — At the beginning of combat on your turn, if you control your commander, put a +1/+1 counter on each creature you control.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
6 printings · $0.35 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2018 | 31 | Uncommon | $0.39 | — |
| Commander Masters | 304 | Uncommon | $0.35 | $0.79 |
| Marvel Super Heroes Commander | 176 | Uncommon | $0.35 | $0.49 |
| Neon Dynasty Commander | 121 | Uncommon | $0.49 | — |
| The List | NEC-121 | Uncommon | — | — |
| Starter Commander Decks | 198 | Uncommon | $0.35 | — |
3 rulings from Wizards of the Coast
- 13 July 2018If you have multiple commanders, you need to control only one for the lieutenant effect to happen.
- 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 don't control your commander as the lieutenant ability resolves, you won't get its effect.