What effect did Pokemon have on the metagame?
With its introduction in DLC2, Ho-oh quickly rose to being one of the most prominent Defog users in the tier. With its great typing, fantastic special defense, access to Regenerator, and now Heavy-Duty Boots, Ho-oh was used on balance and stall teams to great effect. In my opinion, Ho-oh signified the move of the meta towards more bulky offense team, filling that hole teams had of a specially-defensive, bulky Defog user with great longevity. Ho-oh was able to check many threatening attackers across the board, such as Regieleki (without Wild Charge), Pheromosa, Magearna, Lunala (if its popped Meteor Beam already), Kartana, and Solgaleo (without Wild Charge). As you can see a decent amount of these Pokemon have prerequisites attached, but that is due to them having to tech new moves in order to now bypass Ho-oh.
In what main roles was Pokemon used?
It had two roles, one offensive and one defensive. Defensively, it was used as a Defog user with Sacred Fire and Defog, and then two of Earthquake/Whirlwind/Brave Bird/Toxic/Roost. Offensively, it could run Choice Band, Life Orb, or Curse. All of these sets were threatening, but what version you went with was largely personal preference.
What caused it to have a significant impact?
It was fat and it lived a long time, really. It was repeatedly able to switch in over the course of a match and was very hard to wear down.
How do/did you deal with this Pokemon in MnM?
There were three main ways of wearing down Ho-oh. First, by statusing it. A Ho-oh that is poisoned or paralyzed is much easier to deal with, making it harder for it to fulfill its role. The second was by removing its boots. Although Knock off is rare in MnM, pokemon such as Kartana are able to effectively bait in Ho-oh and remove its boots. The third way was to tech coverage for it. Wild Charge Regieleki and Wild Charge Solgaleo became more prominent due to their ability to effectively bring Ho-oh in and remove it with these moves.