Is there a term for the part of a word that "pluralizes" it?

For example, what is the "s" in "apples" or the "ies" in "ponies" called? I found that "morpheme" is probably close, since that's just the smallest grammatical unit in a language, but that's not really identifying the pluralizing nature of the morpheme itself.

I've suggested "pluralizer" to a friend, but that kinda seems made up and I was wondering if there is an actual grammatical term.

This is usually just called the "plural marker" or the "plural morpheme". In English it's usually "-s" though there are plenty of exceptions, and other languages of course have their own plural markers.

Pluralizer

https://en.oxforddictionaries.com/definition/pluralizer

Grammar. A suffix, prefix, inflection, or auxiliary word which forms a plural.

Your original suggestion to your friend is totally sufficient, as it turns out. It does feel slightly made up!