A tricorn was a type of hat.
The Second Doctor once wore a British Army tricorn as a disguise. (TV: The Highlanders)
Avatar wore a tricorn hat. (PROSE: Of the City of the Saved...)
One of the Doctor's incarnations predating the First Doctor wore a tricorn. (TV: The Brain of Morbius) The Eighth Doctor later found his old tricorn in the TARDIS secondary control room, reminiscing that it had been "ages" since he'd worn it. (TV: The Brain of Morbius)