Godzilla - Zilla on the same style as the other monsters in the new movies.

But he wasn't a huge coward, he had survival instincts, he obviously was not strong enough for the Big G. so he take a chance and ran away, but later on he helped Godzilla and other monsters to take down some alien kaiju invasion:

You could actually reintroduce Zilla into the Monster verse (Or any Godzilla movie) and use it as a strong opponent if you use it correctly, Zilla must be a swarming enemy, Zilla on its own is the weakest interpretation of Godzilla (The main reason why he is so hated on Japan), so much so that is even possible to take it down with regular weaponry, what you can do is to use is OP reproductive capacity to its favor, for instance, I can see Godzilla taking off easy a single Zilla but struggling against a hundred or a thousand Zillas.