sui generis

sui generis \soo-eye-JEN-ur-us; soo-ee-\, adjective:

Being the only example of its kind; constituting a class of its own; unique.

This man, in fact, was sui generis, a true original.
-- Ruth Lord, Henry F. du Pont and Winterthur

They're a special case, a category of their own, sui generis.
-- Eric Kraft, Leaving Small's Hotel

William Randolph Hearst did not speak often of his father. He preferred to think of himself as sui generis and self-created, which in many ways he was.
-- David Nasaw, The Chief

Sui generis is from Latin, literally meaning "of its own kind": sui, "of its own" + generis, genitive form of genus, "kind."