"And if an ox gore" — the Torah mentions only an ox. But what about other animals? If a donkey kicks someone, or a camel bites, do the same laws apply? The Mekhilta says yes, and derives this through a verbal analogy.
The word "ox" appears here, and the same word "ox" appears in the second Decalogue (Deuteronomy 5:14), in the context of Sabbath rest: "your ox and your donkey and all of your beasts." In that verse, "ox" is explicitly equated with "all of your beasts." Since the Torah uses the same word in both contexts, the Mekhilta transfers the inclusive meaning: just as the "ox" of Sinai encompasses all beasts, the "ox" in the goring law encompasses all beasts as well.
This is a classic gezeirah shavah — a verbal bridge between two passages built on a single shared word. The Torah chose to use "ox" as its example in the goring laws not because only oxen are covered but because "ox" carried an established broader meaning from its use at Sinai. The specific word unlocks a universal application.
The practical impact is enormous. Without this derivation, animal injury laws would apply only to oxen. Owners of horses, donkeys, dogs, and every other animal would face no legal liability when their animals harmed people. One word — "ox" — appearing in two strategic locations, extended the entire body of animal tort law to cover every domesticated creature.