To unlock the "Dark Horse" in FishVille, you'll have reach Level 10 in Mafia Wars. Once that's done, you can buy it from the FishVille store for 60,000 Coins (or 54,000 Coins if you're subscribed as a Club Member to the FishVille newsletter). It's also available for 15 Sand Dollars (or 14, if you're a Club Member).
The "Dark Horse" counts as a decoration, not an item, so it won't take up your fish limit. It can also be sold back to the store for 5,000 Coins.