Merriam-Webster has it as a less common spelling.

https://www.merriam-webster.com/dictionary/flunkie

I've seen it as "flunkie" (singular) as well, though it may not be standard.

I've never seen it as that either. (So neither "flunkey" nor "flunkie".)

AFAIK it's "flunky". (Plural "flunkies")