THK got it, and he wins this round.
And in hindsight, yes, I admit this may have been a little too hard...the pattern, as most of you have figured out, was that Moobles like words which don't have the letter 'S' in their plural form. I actually had to look up words like "Jeep" and "onomatopoeia" to make certain about their plural forms...
Anyhow, it's THK's turn now.