Cheapest priced copy (Magic 2011): $0.35 at Card Kingdom, near mint, their price list as of .
Harbor Serpent 
Creature — Serpent · 5/5 · Common · Magic 2012 · #56
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
This creature can't attack unless there are five or more Islands on the battlefield.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
3 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Magic 2011 | 56 | Common | $0.35 | $0.49 |
| Magic 2012 | 56 | Common | $0.35 | $0.49 |
| Magic 2013 | 53 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 15 August 2010The second ability checks how many Islands are on the battlefield (regardless of who controls them) only as attackers are declared. Once Harbor Serpent is declared as an attacker, it will continue to attack even if the number of Islands on the battlefield falls below five.
- 15 August 2010Harbor Serpent's abilities care about lands with the land type Island, not necessarily lands named Island.