The rain in Spain stays mainly on the plain
Mainly rain, if you're in Spain.
The rain in Spain stays mainly in the plains.
The rain on the plane falls mainly in Spain.
My Fair Lady
The rain on the plane falls mainly in Spain.
Cows nibble the flowers or geese wheel above the lake
It does not! That phrase is the name of a song from the musical My Fair Lady. The rainiest part of Spain is in the hilly northwest on the Bay of Biscay.
No, it is an "assonance". Assonance repeats vow sounds, alliteration repeats consonant sounds as in "Peter picked a peck of pickled peppers".
The rain in Spain stays mainly in the Plains. Last year was the hottest ever recorded.
The Rain in Spain was created in 1956.
* "that dolphin-torn, that gong-tormented sea" (William Butler Yeats).