[slantyyz]

And the version of the slur you're talking about is a pretty old one that I have not heard used in decades (of course that may be regional, generational, etc.)

But like you, I have never heard "mongo" used in the same pejorative context.

[jeltz]

It might be rare in English but it is very common in other languages.

[skinkestek]

It is a very common slur among kids in Norway and originally means someone who has the "mongoloid" syndrome. These days it just means "you idiot" after "pro choice" succeeded where Hitler "failed" and got rid of most people with downs syndrome.