From the Latin peculatus—misappropriation of property. Somehow the word "misappropriation" makes the whole act sound a bit nicer.
"Oops! I didn't realize that cow was on your side of the fence! Sorry!"
It's no accident that I used the example of a cow; the Latin pecu means cattle, and pecus refers to livestock. There was once a great property value in animals (not that there isn't now; there's just less livestock ownership).
I can imagine a word like PECULATION referring to the underhanded procurement of someone else's livestock. I can visualize it in a legal document:
________ was found guilty of the PECULATION of six cattle from the property of ________, on 23 May 1826. ________ is hereby sentenced to _________.
Something like that.
HOWEVER, the close and perhaps more modern synonym, embezzlement, does not strike me as appropriate a term for the villainous movement of livestock. Embezzlement implies the deceitful re-appropriation (I think re- is more effective than mis-) of something intangible—mostly money, as transferred among various bank accounts. One doesn't embezzle cash, he embezzles stocks or bonds or account balances.
PECULATION, therefore, implies the physical. For example:
She PECULATED the lawn gnome from the neighbor's house at 3 a.m.
The interesting thing about both of these words is that they suggest a post-re-appropriation insistence on original ownership:
"I had that money all along."
"I've had that lawn gnome since last spring. I don't know what happened to yours."
I mean, this is just my theory based on the Latin lineage. PECULATION in "reality" is just another way of saying embezzlement. But that's boring. We need a better word than "stealing" or "theft" for the cunning re-appropriation of livestock. Or lawn ornaments.