Technically speaking leather can be made from any animal hide. We tend to make it from bovines because they have a lot of skin, which means it’s easier to make big pieces. They’re also domesticated, which makes it easier to make a lot of leather at once.
The sacrifice doesn’t surprise me, humans don’t even need religious festivals to celebrate slaughtering each other. All we need is a reason why the people we killed “don’t count” as humans. Example: they live in the wrong country, or were born to the wrong parents, or looked at us funny, or we needed to foster deep-seated hatred that would result in a future insurgent cell so we have continued reason to build armaments to do it again.
In this case it is likely there’s something like the scenario in Wicked: Tauros are “Animals”, sentient beings. The festival cow is an “animal”, a being with no or limited consciousness that hasn’t been deemed the bearer of a soul. Most cultures only consider a killing “murder” if a being with a soul is losing its life. Definitions of “having a soul” vary to suit individual agendas.