According to "Dictionary of Selected Synonyms in
the principal IE languages" (C.D.Duck)
Latin caballus may mean "castrated"
cf.
Grk kabe:los "castrated"
Slavic kobila or kobyla "mare"
note: (i) = short i (u) = short
u
ChSlav konj(i) < *komniyo- cf. Russian
komon' VPruss camnet Lit kumelys (foal), kumele (mare)
*komnijo < *kobniyo- ?