That's strange. You could breed a Grookey with a female Rillaboom which knows Knock Off and a male Venusaur with Leech Seed. Then you breed a Ferrothorn with that Grookey (but make sure is male) then you should get it. I don't play the in game series so I don't know if this is true or not. Note that using that set isn't competitive, here is a recommended set:
Ferrothorn @ Rocky Helmet
Ability: Iron Barbs
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
IVs: 0 Atk
- Toxic
- Leech Seed
- Stealth Rock
- Spikes
Because this isn't confirmed, I didn't put it as an answer.