Which bosses can only be fought in certain endings?

The only ending that has any extra bosses is the Shura Ending where you need to side with Owl and then you fight Emma and Lord Isshin.

In the other 3 endings you need to side with Kuro and defeat Owl and then mostly collect some items or trigger some events. In the Purification Ending, you need to defeat Owl a second time in the Hirata Estate which I guess counts as an extra boss.

The different endings are mostly about doing some extra leg work and not much to do with extra bosses.