As quoted from Business Wire,
“Warner Bros. and Legendary released Godzilla in May 2014 with an agreement to release Godzilla 2 on June 8, 2018. Both films feature the human Monarch organization. Shortly following Legendary’s pact with NBC Universal, Legendary acquired rights to additional classic characters from Toho’s Godzilla universe, including Rodan, Mothra, and King Ghidorah. This paved the way for developing a franchise centered around Monarch and anchored by Godzilla, King Kong, and other famous creatures.
When Legendary announced films centered on Godzilla and Kong, fans all over the world speculated these two characters might one day meet in the same film. Classic Toho monsters including King Ghidorah, Mothra, and Rodan, as announced at Comic-Con 2014, may also join the Legendary pantheon of giant monster mayhem going forward.”