The War of the Gargantuas

Furankenshutain no kaijû: Sanda tai Gaira is a movie starring Russ Tamblyn, Kumi Mizuno, and Kenji Sahara. An experimental lab animal called a gargantua escapes from his captors and is suspected to be the creature that is killing...
Gaira, a humanoid sea beast spawned from the discarded cells of Frankenstein's monster, attacks the shores of Tokyo. While the Japanese military prepares to take action, Gaira's Gargantua brother, Sanda, descends from the mountains to defend his kin. A battle between good and evil ensues, leaving brothers divided and a city in ruins.