It's a Shellder.
I just realized that.
Actually, the anime implies Shellder just have the hots for the King's Rock, and when a Slowpoke puts it on its head, one will bite it.
But the reason it can make Pokemon flinch is because when the holder attacks, the Shellder pops out and scares them. It makes perfect sense! And game-wise, do the Shellder just latch on right before the Slowpoke gets traded, or does the Shellder get fused with the Slowpoke during the trade? Which makes more sense?
No it's not.
In-game picture of King's Rock:
Doesn't look like Shellder in the slightest.
It cannot be a Shellder because Poliwhirl evolving into Politoed has nothing to do with Shellder. The king's Rock probably attracts Shellder to the Slowpoke's head.