Here is the manual section listing all the "Games and Amusements" that ship with Emacs by default:
https://www.gnu.org/software/emacs/manual/html_node/emacs/Am...
There's also nyan-mode, made by yours truly, but unfortunately it does not ship with Emacs by default :).
Was surprised how high quality the repo is, too!
https://github.com/TeMPOraL/nyan-mode
It even has an accompanying blog post, which I found informative. http://jacek.zlydach.pl/blog/2011-08-25-introducing-nyan-mod...
And even a Show HN. https://news.ycombinator.com/item?id=2906632
As for getting it into mainline, I'd be OK with going through the FSF copyright paperwork, but I worry this will probably fail over the uncertain status of Nyan Cat image/music IP...
That’s how MooGNU was born: https://archive.org/details/M00GNU
How many other meme modes exist? I dug up the earliest commits I could find, and apparently one of the earliest hooks was "protect-innocence-hook": https://twitter.com/theshawwn/status/1327459335245422597
If the buffer's filename is "sex.6", it asks whether the user is over 18. If so, it renames the buffer to "celibacy.1".
Apparently it was in the file lisp/play/meese.el. Emacs archaeology is more fun than it should be.