Cheapest priced copy (Modern Horizons 2): $0.35 at Card Kingdom, near mint, their price list as of .
Arcbound Tracker 
Artifact Creature — Dog · 0/0 · Common · Modern Horizons 2 · #112
Menace
Modular 2 (This creature enters with two +1/+1 counters on it. When it dies, you may put its +1/+1 counters on target artifact creature.)
Whenever you cast a spell other than your first spell each turn, put a +1/+1 counter on this creature.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Modern Horizons 2 | 112 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 18 June 2021Arcbound Tracker will consider spells you cast before it was on the battlefield. For example, if Arcbound Tracker is the first spell you cast in a turn, each subsequent spell you cast after it's on the battlefield will cause the last ability to trigger.
- 18 June 2021If this creature gets enough -1/-1 counters put on it to cause it to go to the graveyard, modular will put a number of +1/+1 counters on the target artifact creature equal to the number of +1/+1 counters on this creature before it left the battlefield.