"For how knoweth a man the master whom he has not served (Mosiah 5:13)?" Benjamin comes back to the concept of service at the end of his speech. He did not leave without explaining why we are expected to serve. If we want to know the master, we must serve the master. That is what we get out of service. When we know the master, we belong to the master because we have entered into a covenant with him, so we will not be driven out. In Mosiah 5:14, Benjamin equated the man who had not served and known the master to a familiar animal, one that they may have even seen driven from the temple on the Day of Atonement.
Figure 7 John W. Welch and Greg Welch, "Three Steps in Benjamin’s Logic on Service," in Charting the Book of Mormon, chart 86.
"Doth a man take an ass which belongs to his neighbor and keep him?" Benjamin said that the man would not allow such an ass to feed among his flocks. He would drive him away and cast him out. Here was an animal being driven out because he did not know the Lord. It was not recognized as an animal of the Lord. We will be driven out just like that animal, carrying out with us all of our own impurities, and have our names blotted out unless we take upon ourselves the name of Christ, remember it, and not transgress the covenant (5:11).
It is worth knowing that on some occasions a donkey was also used as a scape animal (like the scapegoat). Different animals were used. Benjamin here happens to speak of an ass or a donkey. We know from ancient near-eastern materials that some cultures, not the Israelites, occasionally used a dog. The Hittites sometimes used a dog and sometimes they even used a rabbit.