Watch out for Ferrothorn, it's bad speed and high defenses qualify it for an extremely bulky gyro ball/stealth rock nuisance Counters: SpAtk Fire types and Rapid Spinners. Sableye and other Prankster Pokemon, If you see a Sableye fighting a Physical Attacker switch to a SpAtk'er or a Fire type as Will-O-Wisp is affected by Prankster and will go first if they use it. Counters: Fire types or Special Attackers. Speed Boost Substitute Protect Possibly Swords Dance Baton Passers, it sounds dumb but Pokemon like Ninjask play this roll. Counters: Pokemon that have stat increasing moves, they won't attack (and most likely can't) so when they see you using Agility, Swords Dance, Iron Defense, or Nasty Plot they will be forced to Baton Pass or switch soon so you don't get too strong. Pokemon that have same stats such as Mew, due to Mew having 100 base in all stats and it's ability to learn all HMs and TMs it can really mess up a perfect team if you try to read what it will do and don't read right. Counters: Pokemon that can Physically Attack or Specially Attack. There's also obvious stuff like megas but obvious stuff is obvious.
Hope I Helped :D- SwagMeowth