In the accounting of capitalism, animals count as capital with depreciation or maintenance costs in feed or whatever. Slaves also counted as capital and the correct response to slavery was still abolishment - being accounted for as labour or capital is not a moral distinguishment or an aspersion on how valuable or not valuable an animals work is. Much the same way you wouldn't count your tractors "labour" and production over maintenance as "surplus value" you also wouldn't account that way work animals.

Farm animals were some of the first "technologies" - I know I'm instrumentalizing here, but stick with it - and the first things we could do to increase the organic composition of capital was by introducing draft animals to farming or mules for mining etc. - it's also what they did by introducing slavery to cotton picking or whatever too.

For Marx in Estranged Labour there is a distinction between human and animal that he elucidates quite well to help distinguish between what Marx considered human labour and animal labour. You can read it here. The whole thing is an interesting read but here's some vital parts:

In estranging from man (1) nature, and (2) himself, his own active functions, his life activity, estranged labor estranges the species from man. It changes for him the life of the species into a means of individual life. First it estranges the life of the species and individual life, and secondly it makes individual life in its abstract form the purpose of the life of the species, likewise in its abstract and estranged form.

For labor, life activity, productive life itself, appears to man in the first place merely as a means of satisfying a need – the need to maintain physical existence. Yet the productive life is the life of the species. It is life-engendering life. The whole character of a species, its species-character, is contained in the character of its life activity; and free, conscious activity is man’s species-character. Life itself appears only as a means to life.

The animal is immediately one with its life activity. It does not distinguish itself from it. It is its life activity. Man makes his life activity itself the object of his will and of his consciousness. He has conscious life activity. It is not a determination with which he directly merges. Conscious life activity distinguishes man immediately from animal life activity. It is just because of this that he is a species-being. Or it is only because he is a species-being that he is a conscious being, i.e., that his own life is an object for him. Only because of that is his activity free activity. Estranged labor reverses the relationship, so that it is just because man is a conscious being that he makes his life activity, his essential being, a mere means to his existence.

In creating a world of objects by his personal activity, in his work upon inorganic nature, man proves himself a conscious species-being, i.e., as a being that treats the species as his own essential being, or that treats itself as a species-being. Admittedly animals also produce. They build themselves nests, dwellings, like the bees, beavers, ants, etc. But an animal only produces what it immediately needs for itself or its young. It produces one-sidedly, whilst man produces universally. It produces only under the dominion of immediate physical need, whilst man produces even when he is free from physical need and only truly produces in freedom therefrom. An animal produces only itself, whilst man reproduces the whole of nature. An animal’s product belongs immediately to its physical body, whilst man freely confronts his product. An animal forms only in accordance with the standard and the need of the species to which it belongs, whilst man knows how to produce in accordance with the standard of every species, and knows how to apply everywhere the inherent standard to the object. Man therefore also forms objects in accordance with the laws of beauty.

It is just in his work upon the objective world, therefore, that man really proves himself to be a species-being. This production is his active species-life. Through this production, nature appears as his work and his reality. The object of labor is, therefore, the objectification of man’s species-life: for he duplicates himself not only, as in consciousness, intellectually, but also actively, in reality, and therefore he sees himself in a world that he has created. In tearing away from man the object of his production, therefore, estranged labor tears from him his species-life, his real objectivity as a member of the species and transforms his advantage over animals into the disadvantage that his inorganic body, nature, is taken from him.