Green’s Dictionary of Slang

caster n.2

[SE cast off; note milit. jargon caster, a horse considered no longer fit for the cavalry or horse artillery and sold at public auction]

anything or anyone that has been rejected or cast aside.

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J. Lang Wanderings in India 144: The horse which drew the buggy had been a caster... a horse considered no longer fit for the cavalry or horse artillery, and sold by public auction [...] [M.] [F&H].