E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Seian Horse (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 Cæsar. Antony had the horse next, and after the battle of Actium slew himself.
Like the gold of Tolosa and Hermiones necklace, the Seian or Sejan horse was a fatal possession.