[YeGoblynQueenne]

Thank you! Yeah, that's the one I keep forgetting, though it doesn't end in "omenon".

[blackshaw]

Another one I just remembered: "automaton/automata".

This site lets you search for English words by suffix, and the only words it can find ending in "-omenon" are "phenomenon" and... "superphenomon". What a phenomenal result.

https://www.litscape.com/word_tools/ends_with.php