Altaria should have Cotton Guard as a defensive threat. After a Cotton Guard not even STAB Banded Stone Edges 2HKO it... without using Roost beforehand. I think not even Rhydon could 2HKO it back when he was NU.
That said, it might fall under specially defensive, since Cottontaria goes Sp.Def to maximise overall bulk after a Cotton Guard.
Pretty much the only thing that stops Cotton-Taria is Ice Beams. Meanwhile it can abuse Roost and Roar. You can't Toxic-stall it because of Natural Cure [Although that does remove Cotton Guard], and it's typing is great. Coupled with hazards, such as T-Spikes and SR, CottonTaria can be a nigh unstoppable lynchpin.