"And it shall be with you" — the Mekhilta interprets this as meaning "in your domain." When you find a lost animal, it must be kept in your care, under your control, until its owner is identified and comes to claim it.
"Until drosh your brother" — the word "drosh" means to inquire. You must investigate whether the claimant is actually the owner. Specifically, you must determine whether the person claiming the animal is trustworthy. Can he provide identifying marks? Does he have a reputation for honesty? You do not simply hand over the animal to the first person who shows up.
"Then you shall return it to him" — but only after proper verification. The Mekhilta derives a practical ruling from the return obligation. A found animal that can work — such as an ox or donkey — should be put to work during the period it is in the finder's custody. The animal earns its keep through labor, and the finder is not required to feed it for free.
But an animal that cannot work — or produces nothing of value during its stay — creates a problem. The finder cannot be expected to feed and shelter the animal indefinitely at his own expense. The Mekhilta acknowledges this tension: there can be "a lost animal which works and eats, but not one which is fed without working." The obligation to return lost property does not require the finder to absorb unlimited costs.