Had he not appeared in Ghidorah, the Three-Headed Monster, which established them as friends, Godzilla vs. Rodan may have happened at some point down the line. Due to Rodan's flying ability ...

Mothra and Rodan, for example ... Names Varan, Baradagi, Obake Subtitle Giant Flying Squirrel Monster, Winged Membrane Dragon, Mountain God Designed By Teizo Toshimitsu, Kanju Yagi, Yasuei ...