I think we, certainly I, have a lot of emotional investment in the animals. See them in the parade ring, maybe get to know their "personality" or running style, maybe bet on them so for a few minutes it is "my horse". Then it is horrible seeing them come to grief. So I think it is natural to be upset.
Princess Diana, Peaches etc. never knew, never cared about so to me it would be strange to feel upset about their death. Shame about it but no grief or upset from me really and can’t understand why a million strangers grieve.