"An ass or an ox or a lamb" — the Torah lists three specific animals in the context of deposit law. But the Mekhilta asks: what about all other domesticated animals? Are only these three species covered?
The answer lies in the broader phrase that follows: "all beasts to watch." This general statement encompasses every animal that might be entrusted to a guardian. The specific list of three animals does not limit the law — the general phrase expands it to include all beasts.
This follows a standard Mekhilta interpretive principle: wherever a general statement adds to specific cases, all examples are included within the general rule. So why did the Torah bother listing specific animals at all?
If only the general phrase — "all beasts to watch" — had been stated, one might conclude that the guardian is not liable unless he was entrusted with all types of animals simultaneously. The deposit laws would only activate when someone left their entire herd — donkeys, oxen, and sheep together — in the guardian's care. Leaving just one type of animal would not trigger liability.
The specific list — "an ass or an ox or a lamb" — corrects this. The word "or" between each animal establishes that liability attaches to each species individually. Deposit one donkey, and the guardian is liable for that donkey. The Torah needed both the specific list and the general phrase to construct a complete law: each animal individually, every species included.