Jacob's Ladder

Jacob's Ladders are a unique type of flower in the. They only spawn when the player's town is in perfect condition, and have been in the game since. You will also receive the golden watering can when your town is in perfect condition for 15 days straight, and this is generally a good indicator of when the plant may appear.

This flower does not breed when planted next to other Jacob's Ladders or other flowers, though it does increase the chance of breeding other flowers when planted next to them. The in-game model does not even remotely match the real-life Jacob's Ladder which is a cluster of purple bell-like flowers and is much more reminiscent of the Lily-of-the-Valley.

It is often considered one of the hardest flowers to obtain.

Also the Jacob's Ladder also appears in the newest release, New Leaf. It doesn't seem to only come when your town is 'perfect' but that is only speculation. It looks the same as in Wild World.

Further Information
The real-life flower known as Jacob's Ladder is a medicinal flower. However, the flower seen in Animal Crossing is called the Lily of the Valley in English. In Japan the name スズラン Suzuran is a name given to Lily of the Valley, but when translated to English it came out as Jacob's Ladder, hence the apparent translation error.