What is the plural possessive of beatles?

The noun "Beatles" is a proper noun, the name of a group of British singers/songwritiers. A proper noun is always capitalized.

The possessive form of the plural, proper noun Beatles is Beatles'.

The apostrophe following the ending s indicates possession.

Example: The Beatles' songs are still very popular.

The plural form of the other type of beetle, a Word for a type of insect, is beetles.

The plural possessive form is beetles'.

Example: The beetles' colors are green and brown.