There are some terrifying Pokémon of tremendous power out there, legendary beings that can control the elements and so forth, but earlier in my Poké career, it was a gleeful pink-white-and-black cow that haunted me.
Miltank. One of the most deceptively fiendish pocket monsters you'll encounter in the Johto region.
It's not that far into Pokémon Gold that this mooing monstrosity is summoned by normal-type gym leader Whitney to rain endless frustrations upon you. But Game Freak doesn't care about you being prepared—all they care about is that your Pokémon are ready to die.
She's got 100 base speed and moves like no cow I've ever seen. If you think you're close to KO'ing her, the replenishing move 'Milk Drink' will restore her countless times, and both 'Stomp' and 'Rollout' are the kinds of physical moves that Game Over screens are made of.
Even facing Whitney when overpowered won't guarantee you a win over Miltank, who is a contender for the most maddening Pokémon opponent you'll face in relation to the point of the game they're found.
There are few specific things that have made me think about giving up my Poké travels forever, but Whitney's Miltank is one of them.