Another thing to consider about Terapagos' ability is that, despite it being a better version of Multiscale, all of the other mons with Multiscale/Shadow Shield had access to recovery (Dnite and Lugia have Roost, Lunala has Moonlight) which helped them exploit that mechanic to the fullest. Terapagos, on the other hand, needs to rely on Leftovers+Protect or Wish support to actually use it more than once, and unlike DD Dragonite it lacks the immediate offensive power/Speed to really be threatening once it enters the field. Sure, it might be able to get a free CM, but even then that set is still quite vulnerable to the likes of Encore, Unaware mons, Ting-Lu's Whirlwind/Garchomp's Dragon Tail, Pex's Haze or just simple moves like Toxic and Taunt. It might actually be more effective as a consistent bulky Spinner rather than as a Sweeper itself.
gets up one Nasty Plot on something it forces out, and it's pretty much unstoppable now. Pretty much nothing switches in and beats Darkrai 
getting 2HKOd is just one example, and you'd be risking your one answer by hard switching into the wrong move, Dark resists like Enamorus 
do drop to Psychic. This means you have to resort to revenge killers, after a pokemon has already been brought down, a pokemon forcing this is already overbearing, but Darkrai 
(CC or first switch in Body Press), Clear Body Choice Specs Dragapult 
, Choice Scarf Enamorus 
, Tera Grass Choice Band Rillaboom 
, and Tera Water Ogerpon Wellspring 
(as long as you don't get chipped or flinched)
and Zamazenta 
