Cheapest priced copy (Foundations): $17.99 at Card Kingdom, near mint, their price list as of .
Mossborn Hydra 
Creature — Elemental Hydra · 0/0 · Rare · Foundations · #471
Trample (This creature can deal excess combat damage to the player or planeswalker it's attacking.)
This creature enters with a +1/+1 counter on it.
Landfall — Whenever a land you control enters, double the number of +1/+1 counters on this creature.
Legal inCommanderBrawlStandardPioneerModernLegacyVintage
Not legal inPauper
6 printings · $17.99 to $99.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Foundations | 107 | Rare | $18.99 | $17.99 |
| Foundations | 337 | Rare | $21.99 | $37.99 |
| Foundations | 399 | Rare | — | $99.99 |
| Foundations | 471 | Rare | $17.99 | $24.99 |
| Foundations Promos | 107p | Rare | $18.99 | $19.99 |
| Foundations Promos | 107s | Rare | — | $21.99 |
5 rulings from Wizards of the Coast
- 8 November 2024If Mossborn Hydra somehow enters at the same time as one or more other lands you control, its last ability triggers for each of those lands.
- 8 November 2024A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.
- 8 November 2024To double the number of +1/+1 counters on Mossborn Hydra, put a number of +1/+1 counters on it equal to the number it already has. Other cards that interact with putting counters on it will interact with this effect accordingly.
- 8 November 2024Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).
- 8 November 2024A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.