There a footnote among the discussion of the Caledonian war of Septimius Severus.
That the Caracul of Ossian is the Caracalla of the Roman History is… not without difficulty. In the Caledonian war, the son of Severus was known only by the appellation of Antoninus; and it may seem strange that the Highland bard should describe him by a nickname, invented four years afterwards, scarcely used by the Romans till after the death of that emperor, and seldom employed by the most ancient historians.
Quite. Almost as if Ossian was made up at a (much) later date.