First of all, animals should not be used for work because :im-vegan: . But having said that... I see how human labor creates surplus value. And I think I understand why machines don't create surplus value. But what about animals that get exploited to perform work, say the mule that pulls a plow? Like humans, they require a "real wage" to sustain them i.e. food, shelter, and medical attention, right? And if the value of labor power of a day of a mule is say 2 hours (that's how much human labor is invovlved in making feed, etc), then if this mule works for more than 2 hours, are they not creating surplus value for the capitalist that claims to own them?
It would make a bit more sense to me for animals that we can exploit something their doing without having to give them much in return. Bee's, already being mentioned, is a good example because of honey. Also, any pollinator.
I'm sure that there's smarter people that me who have a better answer for how livestock create surplus value. But from years of having livestock, you've gotta put a decent amount of time/energy/money into keeping them and what they provide that can be sold tends to be valued less and less by customers. So it does makes sense that there's "cash profit" made somewhere, it winds up being not a lot.
If we viewed the "profit" as the stuff the animals produced (or were turned into) and used directly, that would be much easier for me to understand as "exploitable surplus value".