"If it were hired, it came by its hire" — the Torah introduces a fourth category of guardian: the hirer. Someone who rents an animal occupies a middle ground between the unpaid guardian and the paid guardian. The Mekhilta works out where exactly the hirer's liability falls.
One might think that if the hirer swears an oath (as the unpaid guardian does), he is completely exempt from all payment. But the Mekhilta reasons by analogy. A paid guardian receives compensation for his watching and is liable for theft and loss. A hirer also gives compensation (in the form of rental payment) and also benefits from the arrangement. Since both the paid guardian and the hirer benefit financially, they should share the same level of liability.
The conclusion: just as a paid guardian who swears an oath exempts himself from accidental losses but still pays for theft and loss, a hirer also pays for theft and loss. The oath covers accidents — death, breaking, and seizure — but does not shield the hirer from liability when the animal is stolen or goes missing.
The Mekhilta constructs a complete hierarchy of guardian liability. The unpaid guardian: liable only for negligence. The paid guardian: liable for theft and loss. The hirer: same as the paid guardian. The borrower: liable for everything except death during normal use. Each category reflects the degree of benefit the guardian receives from the arrangement.