Rig - A
male horse or
pony who has either one or both testes undescended and
retained, ie retained in the
abdomen.
In
layman's terms: basically, a badly
castrated male horse who has the
hormone balance of a stallion, but is un-fertile for
biological reasons (
body heat rendering the
sperm useless)
See: stallion, gelding, mare, foal, filly, steed.