Cheapest priced copy (Ravnica: Clue Edition): $0.35 at Card Kingdom, near mint, their price list as of .
Territorial Boar 
Creature — Boar · 2/2 · Common · Ravnica Allegiance · #145
Whenever a creature you control with power 4 or greater enters, this creature gets +1/+1 and gains vigilance until end of turn.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Ravnica: Clue Edition | 175 | Common | $0.35 | — |
| Ravnica Allegiance | 145 | Common | $0.35 | $0.49 |
3 rulings from Wizards of the Coast
- 25 January 2019The entering creature must have power 4 or greater as it enters the battlefield, or Territorial Boar’s ability won’t trigger. Static abilities that raise (or lower) a creature’s power are taken into account. However, you can’t have a creature with power 3 or less enter the battlefield, raise its power with a spell, an activated ability, or a triggered ability, and have Territorial Boar’s ability trigger.
- 25 January 2019If the entering creature’s power changes to 3 or less after it has entered the battlefield, Territorial Boar still gets +1/+1 and gains vigilance.
- 25 January 2019If Territorial Boar’s power is raised to 4 or greater as it enters the battlefield, it will cause its own ability to trigger.