Don't use this
Forretress @ Leftovers
Ability: Sturdy
EVs: 248 HP / 8 Def / 252 SpD
Sassy Nature
IVs: 0 Spe
- Rapid Spin
- Spikes / Stealth Rock
- Toxic
- Gyro Ball / Volt Switch
Use this
Ferrothorn @ Leftovers
Ability: Iron Barbs
EVs: 252 HP / 88 Def / 168 SpD
IVs: 0 Spe
Sassy / Relaxed Nature
- Spikes / Stealth Rock
- Leech Seed
- Gyro Ball / Power Whip
- Power Whip / Protect / Thunder Wave
Ferrothorn is a better mixed tank than Forretress, as it offers better special bulk, but does have lower physical bulk but it's a very small difference. Also it has Iron Barbs which gives physical attackers hell, especially if they lack recovery.
The only thing going for Forretress is Rapid Spin, Volt Switch and Toxic Spikes. Forretress isn't a great spinner anymore since it's so easy to set up on, especially with a Substitute to bypass Toxic and plenty of better alternatives are available, such as Starmie or Excadrill. Slow Volt Switches are really good though, but that alone doesn't warrant use of Forretress imo
Toxic Spikes aren't that viable tbh and shouldn't be used over Spikes or SR.
Ferrothorn is nowhere near as passive as Forretress and can easily dent and break Substitutes of neutral/SE threats with Power Whip or Gyro Ball. It also has access to Thunder Wave, which can shut down setup sweepers with ease. Leech Seed is also a good support move for wearing down opponents and can make Ferrothorn difficult to take down with unboosted or neutral moves