Kaiju is strange or ominous plus beast; kai is the same element as in kaii, strange events, and yokai. The film genre took the word in the 1950s, Godzilla in 1954, and the omen-beast became a box office category.
The first kaiju story is true: in 1954 the American Castle Bravo test doused the Japanese tuna boat Lucky Dragon No. 5 with fallout, and its radioman died. Months later Godzilla opened with a fishing boat destroyed by a glowing sea monster. Honda's film is the hydrogen bomb given scales, and the country lined up to attend the memorial.
The claim: the kaiju are the yokai returning through the bomb door, the strange-omen beasts of the Edo ghost-story tradition re-emerging as radioactive giants because the modern age broke the old containment. The genre is kaidan culture with Geiger counters, and the monsters keep arriving because the warning was never heeded.
A word for omen-beasts became a word for rubber-suit giants. What were the omens warning about?