By Rabbi Carl M. Perkins:

Parashat Mishpatim
February 1, 2003
 
 
If you encounter your enemy’s ox or donkey straying,
you must surely return it to him. 
If you see the donkey of one who hates you crouching under its burden, and would refrain from helping him,
you shall nevertheless help, surely help with him. 
Exodus 23: 4-5

Do not look upon the ox of your brother—or his sheep—wandering away
and hide yourself from them.
You are to return, yes, return them to your brother. 
If your brother is not close to you,
or you do not know who he is,
you shall bring it home and it shall remain with you
until your brother’s inquiry.
Then you shall give it back to him. 
You shall do the same with his donkey.
You shall do the same with his garment.
And so too shall you do with anything
that your brother loses and you find:
You must not remain indifferent. Do not look upon your brother’s donkey or ox fallen on the road,
And hide yourself from them.
You must surely help him raise it. 
Deuteronomy 22: 1-4

When it comes to returning a lost animal, it makes no difference whether it is an ox or any other animal, as it is written, … and so shall you do with anything that your brother loses (Deut. 22:3). If so, why does the text refer to an “ox”? Because the text refers to the commonplace. (B. Bava Kamma 54b). (For it is the practice of oxen to graze in the field, and they are likely to wander off and be lost. Torah Temimah) 
Come and hear: If a friend’s donkey requires unloading, and an enemy’s requires loading, one’s first obligation is toward one’s enemy, in order to subdue his evil inclination. (B. Bava Metzia 326) 
If you encounter. I might understand this in its literal sense, yet the next verse says, If you see. But then again, I might understand, If you see, to mean, even at a distance of a mile, but the previous verse says, “If you encounter.”

How then can these passages be maintained? The sages established an arbitrary measure of two fifteenths of a “mile” (i.e., 1,000 cubits), that is to say, a ris (i.e., 133 feet). (Mechilta, Kaspa 2). You shall bring it home (Deut. 22:2). The animal found must be such as is usually brought home. This excludes an injured one. And it shall be with you, that is, in your possession. Until your brother’s inquiry about it. You must find out whether your brother is a deceiver or not. Another interpretation: Until your brother’s inquiry. Until public announcement has been made about its loss. And you shall give it back (literally, restore it) to him. The animal found must be such as pays for its keep by its work, and not one the keep of which would entail a loss.
(Mechilta, Kaspa 2) 

If he finds it in a stable, he has no responsibility toward it; in the street, he is obliged. … If one unloads and loads, unloads and loads, even four or five times, he is still obligated, because it is written, you shall help, surely help with him.
If the owner of the animal went, sat down and said, “Since the obligation rests upon you, if you desire to unload, unload!” he is exempt, because it is said, with him; yet, if the owner was old or infirm, he is required to do it himself.
Rabbi Yosi the Galilean said, If the animal bore more than its proper burden, then a passerby has no obligation toward the owner, because it is written, If you see the donkey of one who hates you crouching under its burden—which means, a burden which it would ordinarily be able to bear.
(M. Bava Metzia 2:10)