pluck, pick, reject, sort, pick out, draft, weed out, cut out, filter out, screen out, more...

keep, preserve, retain, maintain, save

male, female, deformed, diseased, weak, injured, young, old, more...

annual, old, queer, first, silly, selective, total, fresh, famous, final, initial

undesirable horses,   remove the weakest,   to remove rejects,   select or gather,   thin the herd