The yin-yang release masters are the pandas and the sage of six paths soummons and the scroll is from the sage so it should have an animal that is related to yin-yang
Hydra or Phoenix/all birds honestly any flying summon as I have always thought a wind natured person getting toads as a summon was foolish no matter his father before him having it
he could also just summon an elemental who is comprised of all the elements
doesn't he have the same kurama there? why not make it an invocation? if the problem is the seal you can make the summoning a shadow clone jutsu so kurama can also come out....