(The). A possession which invariably brought ill luck with it.
Hence the Latin proverb “Ille homo habet equum Seianum.” Cneius
Seius had an Argive horse, of the breed of Diomed, of a bay
colour and surpassing beauty, but it was fatal to its possessor.
Seius was put to death by Mark Antony. Its next owner, Cornelius
Dolabella, who bought it for 100,000 sesterces, was killed in
Syria during the civil wars. Caius Cassius, who next took
possession of it, perished after the battle of Philippi by the
very sword which stabbed Caesar. Antony had the horse next, and
after the battle of Actium slew himself. Like the gold of Tolosa
and Hermione's necklace, the Seian or Sejan horse was a fatal
possession. Source: Dictionary of Phrase and Fable, E. Cobham
Brewer, 1894 More on Seian Horse from Infoplease: