Interesting. Bulbapedia says it was the equally pointless Nidoran filler. Needless to say, there probably isn’t a snowball’s chance of me or you finding out which site is correct, but either way it’s pretty much the same story; Crap don’t go down on 100s.
I believe it depends on whether or not sites count the two Christmas episodes as part of the regular production run. SPP does.
But people who use Bulbapedia's numbering system have actually argued that the Nidoran filler
does fall in line with the "pattern" since the two Nidorans evolve at the end, even though they're not part of the main team. Believe me, I've disputed this with people.
Finding patterns like this in this show is, if you ask me, bullsh*t. The show may be thoroughly predictable, but in a different sense. Remember when people were complaining about Dawn's name solely because it didn't follow the "pattern" of female leads' names beginning with "M"?
There's probably a mathematical equation out there that proves what it takes to have a firm "pattern" of sorts. It might be three or four numbers minimum, and if one number breaks a steady chain, it's not a pattern. If I have only two numbers - let's say 2 and 4 - I can't seriously prove that the next number will be 6 or 8 from realizing a "pattern" of adding two or squaring it, can I?
Then again, most of the fanbase probably hasn't taken past 2nd grade math.