Morphologically, it should be fer-ok-s (spelled ferox), -s being the nominative singular ending, and the suffix is actually -ok- (spelled -oc-) here.
The OLD (Oxford Latin Dictionary) says the following:
" -ox -ōcis adjl. suff Forms adjs. denoting appreance, as Gk βο-ῶπ-ις, (atrox, ferox); also in wider sense, perh. w. different origins (uelox)."
cf. Weiss 2020: 326
"ferōx 'fierce' (Plaut.+) < 'wild-looking' (ferus 'wild')" with the suffix -ōk-, "originally a root noun from the verbal root *h₃okʷ- 'see, look.' Cf Gk. γλαυκ-ώψ 'with gleaming eyes,' βο-ῶπ-ισ 'ox-eyed'" (cf. Leumann et al. 1977: 377, section 329.4)
Ernout and Meillet are more cautious and say "peut-être" (maybe; s.v. atrox), citing Schmidt 1889 (for Ancient Greek data?).
cf. Walde and Hofmann s.v. atrox.
de Vaan adds that "This can be disputed, but I see no better source for this suffix" (s.v. ferus).