Question Bam! There is a bison!

In theory, any spirit can manifest physically in the physical world. In practice, this happens extremely rarely because of the energy cost. Just being outside of a hallow in the physical world significantly boosts the 'cost of living' for a spirit. Adding in the ability to interact with the physical world pushes that even further. For most spirits, the energy cost set back is not worth the effort.A spirit bonded with a strong avatar is protected and provided for the majority of the time so they can accumulate resources to do this sort of thing in an emergency... and can retreat back to a hallow they know is available to them, the moment their energy burns low.

That said... Tatanka is a special case.

And Kodiak and Grizzly are... doing something different. Remember, those two are 'not' typical spirits. They are powerful sentient beings who found a way to survive a world ending space-time disrupting cataclysm and millions of intervening years.