The Catti or Cat tribe appear to have been the ancient inhabitants of Shetland, or at least the earliest known inhabitants of the islands.
One of the ancient names for Shetland, used in Irish annals was Innse Catt, which seems to mean "Islands of the Cats". Since no species of cat is indigenous to Shetland, this presumably refers to the Catti or Cat People.
The tribe's totem, the cat, is probably a wildcat. The lynx has also been suggested - it became extinct in mainland Scotland round about the Roman period, and could easily be confused with a wildcat in folk memory, even if lynxes tend to be larger and have distinctive ears and tails. Roman military officers would sometimes wear the skin of the lynx (and also lion) around them.
The Catti appear to be commemorated elsewhere. Caithness (Norse: Katanes) presumably takes its name from the tribe. Sutherland, particularly the eastern part is known as Cataibh in Gaelic (confusingly the Gaelic for Caithness is Gallaibh.)
Not a great deal is known about the Catti. It is possible that Clan Chattan, one of the federal Highland clans is a later outgrowth of the group. There are also various other surnames, such as Catto and Catanach which may have a connection to the ancient Catti.