There are four classes of people who are entrusted with items of value (Bava Metzia 7:8, Shevuot 8:1): one who looks after it as a favour, one who does so for a fee, one who borrows it and one who pays money to rent/hire it. Of these four, the one who looks after it as a favour to the owner is the least responsible for what subsequently happens to it, but his having been entrusted with it still denotes some responsibility.
The Mishna (Shevuot 8:3) discusses a case in which a man's ox is being looked after as a favour, but when the owner asks to get it back the guardian swears that it went missing. Witnesses are brought and it is demonstrated that the guardian had actually eaten the ox, making him liable to the owner for the ox's value. Such is not the case if the witnesses testify to his having "stolen" the ox, in which case he is liable to the owner for twice the ox's value, in accordance with the laws of making restitution for theft. Should they testify that he not only stole the ox but that he also slaughtered it, he is liable to the owner for five times the ox's value.
These laws are discussed in more detail in Bava Qama 7 (and a similar scenario is brought in Bava Qama 9:7-8), but the circumstances in this particular mishna are strange to me. A guardian who is discovered to have eaten the ox is liable for the principal only, while one who is discovered to have "stolen" the ox and to have slaughtered it is liable for five times that amount. My question is: how is the consumption of somebody else's ox not in itself an act of theft? Furthermore, how is it possible to consume somebody else's ox without having slaughtered it first?