I think that it is much more likely that Caballus
is a loanword from Celtic and that the Celtic word comes from PIE *kapho-/*kopho
"hoof" via a suffixed form, *kaph-lo "hoofed one" (with -p- becoming -b- before
a liquid -l- instead of being lost, as is usually the case in Celtic). A similar
process is behind Gaulish gabros "goat" (from PIE *kaper, with loss of -e-, an
uncommon shift of -k- to -g-, and -p- to -b- next to the liquid -r-). As a note,
it may be that the Celtic word for sheep, *cair- (Welsh caer, Irish caor,
Gaulish Caera-), comes from *kaper as well, with loss of internal -p- due to the
retention of vowel length in the final -e-.
-C. Gwinn