"An if an ox gore": This tells me only of an ox. Whence do I derive (the same for) all beasts? It follows, viz.: It is written here "ox," and elsewhere (in the second Decalogue, Devarim 5:14) "ox." Just as the "ox" of Sinai is equated with "all of your beasts," so, here, all beasts are to be equated with ox.
If an Ox Gore, the Torah Mentions Only an Ox
Mekhilta Tractate Nezikin 10:2