The English words bear and horse come from old Norse / Indo-European words meaning brown and fast. Languages do that all the time. We even do that for people, like Inuit and Ainu which mean human in their respective languages. Like “what are you?” “Human” “Ainu, got it”
For bears, that works (especially since we know about other words like arctus etc.), but it’s odd that this was also done for other words like horse, linked to PIE “to run”. So yes for the euphemism theory and we consider that calling a cat species " Mau" because of their sound as a euphemism, or horses as a whole “run” because they’re hella fast, but I don’t know if we need to push that further with the taboo theory.