> Someone took a Cyrillic adjective, treated as an English noun, and then applied an English pluralization to it.

So a Russian with a deep background in linguistics intentionally bastardized something so it looks like an English speaker trying to sound Russian. Clever.

An actual English speaker wouldn't even know the root word existed.

Not necessarily, you're overthinking this. It's just what anyone with basic vernacular knowledge of English but no actual Russian language skills would do to generate 'pseudo-Russian'. It's a pretty common idiom to make an English adjective into a Russian sounding one by adding '-ski' to the end of the word, and then to pluralise it, add -s to that, as in normal English.

So, for many small Russian cats, we have the following:

    'Kitten' -> 'Kittenski' -> 'Kittenskis'

The definition of the '-ski' [1] suffix has some other examples. This advert [2] for yoghurt (and yes, it was for Ski unfortunately, and somehow they spent GBP 8.5 million on it...) from 2003 also used the same formulation. See also 'Eye Dialect' [3] used to show foreign or regional speakers in literature, or the use of a backwards 'R' character to create a faux cyrillic, Russian feel; as seen in the Tetris logo.

    1. https://en.wiktionary.org/wiki/-ski
    2. http://www.campaignlive.co.uk/article/170244/nestle-comes-faux-russian-relaunches-ski
    3. https://en.wikipedia.org/wiki/Eye_dialect

I'm delighted that this whole thing led me to a discussion of 'eye dialect'. I've been fascinated by that sort of writing for years, and I never knew it had an actual name.

Sure they would, it was in every silly American movie about evil Russians in the 80s.