If the first creature targeted by Rimehorn Aurochs can’t block the second targeted creature (for example, because the second creature has flying and the first doesn’t, or because both creatures are controlled by the same player), the ability does nothing and the first creature is free to block whichever creature its controller chooses, or block no creatures at all.

Abilities of Aurochs care about other creatures that have creature type Aurochs, which includes (but is not limited to) the Ice Age card namedAurochs.