RBY
Clefable: Can act as both a decent normal-type physical attacker and status spreader. Chansey and, to an extent, Alakazam, who are usually the pokemon that end up absorbing status attacks from the special based pokemon that spread them, have a hard time dealing with Clefable's decent powered STAB Body Slam and Hyper Beam. Unlike other normal attackers like Tauros, Snorlax and Persian, Clefable can spread paralysis directly and can (inaccurately) induce sleep too. Unlike Zapdos and Jolteon, Clefable can Blizzard to 2HKO rocks, and can also use Thunderbolt to do more damage to Starmie, Cloyster and Lapras. Clefable is a kind of all trades master or none Pokemon and suffers from 4MS, but from experience I can tell it can be a pretty dangerous Pokemon, since it can play the role of nearly everyone, to a much lesser extent obviously.
Tangela: Growth, Mega Drain, Reflect, Rest. If it gets going, only critical hit ice attacks or maybe Persian's slash will be able to break it. Unfortunately, it's prone to being stalled out since mega drain at 999 special doesn't even 2hko alakazam. Exeggutor will also be a pain to take down since at least it can contribute to wear down Mega Drain PP effectively. Gengar should be a bit easier but still... And then Zapdos is the ultimate counter, since a crit Drill Peck will do over 90% to tangela. In general, it's better used late game I guess; it's also very hard to set everything up too, so better make sure first that stuff like Tauros is paralyzed. All in all, the ultimate obscure niche Pokemon