Cocoa Island (known as Tails Island or Kokko Island in some media) is the setting for the 1995 Game Gear game Tails Adventure. The entirety of the game takes place there; but Cocoa Island has neither been seen nor mentioned in any subsequent game.
In the opening sequence of the Tails Adventure, the Battle Kukku Army invades the island in full military force, intent on seizing the Chaos Emeralds (which have presumably manifested there due to some inscrutible quirk of Chaos Physics). At the opening of their blitzkrieg attack, the Battle Kukkus burn Poloy Forest to the ground - just as young Miles Prower is taking a nap there. Thus commences the fox's solo adventure!
The island is presumably located somewhere not too far removed from Westside Island and South Island - if only because Tails would have difficulty meeting up with Sonic for their subsequent adventures were Cocoa Island on the other side of the world.
Unlike many of the ubiquitous isles of the Sonic series, Cocoa Island seems to have a fairly normal and homogenous climate, rather than cramming both scorching deserts and frozen wastes within a single landmass. So Cocoa Island's surface is mostly forests, with a mountain or two thrown in. However, Tails dosn't stay on the surface - Tails Adventure explores underground, underwater, and offshore too.
Years before Tails got his gloved hands on property in Mystic Ruins, Emerald Town, or Central City, the fox's first workshop took the form of a modest little abode on the south shore of Cocoa Island. This cosy wood-panneled house has a rather more high-tech attachment in the form of the Sea Fox's sumbarine lab. Tails stores the equipment he can't carry with him here, and returns to the house over and over again during the course of the adventure to refit. Exactly why the Battle Kukku Army dosn't just bomb the place into the ground is anyone's guess; presumably Tails' House is sufficiently inconspicuous as to avoid attracting the eyes (and guns) of the avian imperialists.
Playable stages on Cocoa Island proper (represented by the black entrances on the map) are, clockwise from Tails' House:
- Poloy Forest, set alight by the Kukkus at the outset
- Caron Forest, untorched woodland with caves and waterfalls
- Polly Mountain 2, pitch-black caves and high bluffs
- Polly Mountain 1, another (slightly less dark) entrance to the mountain chain
- Volcanic Tunnel, magma tubes below the surface
- And also Lake Crystal, a Sea Fox level that sees Tails navigate the island's central river.
- Lake Rocky (or "Rocky's Sea"), accessed from the little pier south of Tails' House
- Green Island, the northwest wooded islet; full of bees
- Cavern Island in the northeast; a labyrinth of submarine riptides
- Battle Fortress in the southeast; the Battle Kukku Army's massive command fortress