What does low mean here?
It is also known as the common shoulder muscle, particularly in lower animals (e.g., in domestic cats). Deltoid is also further shortened in slang as “delt”. The plural forms of all three incarnations are deltoidei, deltoids and delts.
oxford dictionaries says “animals of relatively simple or primitive characteristics as contrasted with humans or with more advanced animals such as mammals or vertebrates”.
They are “lower” in terms of their “biological complexity”, e.g. apes are higher animals, worms are lower animals
also: lower animals have a very simple nervous system
higher animals have developed eyes, a more complex brain and so on