Volcarona
Volcarona threatens fat grass types with its STABs and can even KO bulky Lando-T at +1. Skarmory and Mega Scizor are both terrified of Volc. Every one of those walls Dugtrio easily, so Volc's ability to crush them is a major boon to Dugtrio.
Volcarona tears through many offense builds currently, which can be troublesome to Dugtrio. Most importantly, it is a wincon that Dugtrio enables to sweep.
Here's some pests that Dugtrio can erase:
Zard X
Toxapex
Tyranitar
Heatran
Chansey
Scarf Terrakion
Choice-locked Ash-Greninja
Unaware Clefable
Volcarona and Dugtrio synergize well because of their "Zard-Y/BandTar" complex, in which Volcarona pressures the opponent greatly and forces a switch into a counter, which can then be trapped and removed. Suddenly the opponent has no more switchins, and Volcarona QD's and wins from then on. Volcarona can switch into plenty of Pokemon that scare Dugtrio, and Dugtrio comes in against mons that Volcarona fears (see the list above).
Dugtrio's ability to trap mons like Heatran and wear down mons like Toxapex enables Volcarona to run something besides HP Ground, so a possible set could be:
Volcarona @ Firium Z
Ability: Flame Body
EVs: 252 SpA/4 SpD/252 Spe
Timid Nature
-Quiver Dance
-Fire Blast
-Giga Drain
-Roost/HP Ice/U-Turn for Duggy shenanigans/Will-O-Wisp for the tech... you get the picture.
This core needs a sturdy hazard remover and a better defensive backbone, so something like Zapdos or Tapu Fini would work well.
