Mossmane Panthers unlock at Castle LVL. 8
Tricky to set up properly due to them being so fragile, but no troop compares to their speed. With a decent Titan boosting them particularly in toughness, a pure Panther squad can be a fast, lethal force. On attack they can be rather expensive due to their high tendency to get fatally wounded, and defending farm/mine islands with them can be risky; they have a tendency to be summoned piece-meal and with lower numbers a good Ranger/Grenadier squad can systematically shoot them down without too much trouble.
Defending your main Castle with them is a different story, where they are all summoned on the field simultaneously. With a good Titan backing them, any attacker seeing a massive heavily boosted squad of these pelting at them full tilt will definitely get quite nervous, as this can be a very difficult force to counter.