Before there was anyone else in the severed floor, there were goats
When Lumon first conceptualized the experiment design for severance, before testing on humans, they did animal trials. This is common for medical research, especially if the intervention is invasive.
Interestingly, goats are highly social animals and, although not as mainstream as rats, IRL there are quite a few studies on their response to separation/isolation, so it makes total sense to use goats to test severance.
All Lumon experiments are probably trialed on goats before humans. I won't be surprised if the goats were also being tested on the Testing Floor before they introduced human subjects.
So it makes sense to house the goats being used for experiments and this is how the Mammalian Nurturable Services Department came to be - Most likely the first severed department to ever exist.
It's also why the goat was the chosen symbolic sacrifice once Cold Harbor was completed. Goats have been key to the development of Lumon's flagship service/product.