[Sunred]

We all know Den-O's forms are based off Japanese folklore, but some do not know why Zeronos is based off a bull and has a avian imagin(Deneb) as a partner.

Well, since this can be spoilery I will have this covered:

Zeronos is based off the Asian folklore behind Altair, Vega, and Deneb, a folklore known in Japan as Hikoboshi and Orihime.

The story of Hikoboshi and Orihime is a somewhat tragedy about two lovers.
Hikoboshi was an excellent cow herder and Orihime was an excellent weaver.
The Jade Emperor thought these two would make a fine couple and arranged their marriage.
But after their marriage, they were so much in love with each other that they became lazy with their field of work.
The Jade Emperor was furious and separated them to polar opposite ends of the milky way.
This saddened the two that their tears became rain and was enough to form a flood.
To prevent anymore damage, the magpies and crows teamed up to form a makeshift bridge by herding together allowing Hikoboshi and Orihime to reunite temporarily once again and then go back to their respective places until their next reunion.

So as we can see, the concept of lovers that can not be together was the background for Yuto and Airi's relationship.
Deneb is based off the magpies and crows(another motif that is not related to this folklore that Deneb has is that of Musashibo Benkei, a monk).
Altair form's bull motif is a nod to Hikoboshi's cattle.

BTW, Hikoboshi is Altair, the magpies and crows are represented by Deneb, and Vega is Orihime.