HMS UNICORN was designed as one of the last of the successful Leda class frigates, 150 feet long in the hull and principally armed with a combination of 18 pounder long guns and 32 pounder carronades. Their lines were based on a French frigate, the HEBE, captured in 1782, and the whole class was one of the best of the age. It included such fine ships as the famous SHANNON, which captured the American CHESAPEAKE, and HMS TRINCOMALEE which was recently extensively reconstructed at Hartlepool, and which is the only ship afloat in Britain older than UNICORN.

The class was originally rated 38 guns, but under the new establishment of 1817 they were all re-rated as 46 gun frigates, to reflect the importance of their carronade armament.