How much weight could a slave carry? Could a slave pull a wagon or plow a field? Could soldiers mount slaves and charge like cavalry? For that matter do we measure the out put of a combustion engine in slave power or horse power?
Good questions, but irrelevant. Slaves were a traded commodity, and as such were in fact very valuable, value based on training, and abilities.
Please do some research on comparative prices of slaves, say in new Orleans, or elsewhere, you will find they were in the 900 to 1500 dollar range, in 1860's dollars.
More refined ones brought a lot more.
Compared to horse flesh, which could be had at a much lower price, a fraction of the cost.
It was what it was. Arguing that it was not will not change it.