"The story is never the same twice, and parts grow uncertain, or they become suddenly clear in the telling."
This article contains information mainly from the 2007field guide, Secrets of the Clans. While based on canonical information, the book has been repeatedly retconned as the series progressed, and thus its credibility is questioned.
LeopardClan is one of the ancient Clans, along with LionClan and TigerClan. It consisted of leopards who were the swiftest of the great cats. They were huge and golden, spotted with black pawprints. All cats possess their speed and hunting talent due to the characteristics of LeopardClan.
Graypaw mentions LeopardClan when talking to Firepaw about the three great Clans of ancient times. Graypaw accidentally calls it SpottyClan, forgetting the real name. But, knowing they had spots on them which lead him to believe it had something to do with spots. Lionheart, a senior warrior of ThunderClan, then corrects him, knowing he means LeopardClan. He tells Graypaw and Firepaw about their speed and hunting skills, possessing these skills, and about their description.
When Brightsky dies giving birth to her and Mudfur'skits, one survives. Mudfur names her Leopardkit after LeopardClan, hoping it would give her all the strength she would need to survive without Brightsky.
The legend "How LeopardClan Won The River" tells of how the Clan got control of the river in the forest. During a Gathering, the LeopardClan warrior Fleetfoot offered to kill a huge, dangerous boar, Rage, that was loose in the forest. Goldenstar, LionClan's leader, and Shadestar, TigerClan's leader, agreed to give LeopardClan the river for a moon if Fleetfoot succeeded. However, they did not tell Fleetfoot about Rage's even fiercer mate, Fury, who rarely left her nest.
Fleetfoot found and killed Rage with relative ease, only to be confronted by Fury. After a long battle that spanned two days and nights, Fleetfoot defeated Fury by causing her to fall into the river, where she drowned. To make up for their deceit, Shadestar and Goldenstar decided to give LeopardClan sole hunting rights to the river forever.
"LeopardClan were the swift; they had black spots on their pelts like racing pawprints."