Nominating Keldeo, -->XY-era <--
What effect did Keldeo have on the metagame?
Keldeo went on offensive teams and hyper-offense teams. Keldeo primarily rose to prominence in its ability to check mega-kangaskhan, and you would use some sort of resistant switch-in (aegislash) to help pivot your keldeo in to beat kangas. Keldeo also checked Landorus-T. Finally, it was good against things weak to its stabs since it was so fast: hydreigon, heatran, kyurem-b.
In what main roles was Keldeo used?
Being a 108 base speed pokemon with fighting stab made sure that keldeo could outrun and hit the 5 types weak to fighting. The competition it had for that role was terrakion, but terrakion did not beat landorus-t (instead it beat talonflame and charizard-y), and because of how much more common landorus-t was than the fire birds, keldeo was often chosen over terrakion.
What caused it to have a significant impact?
108 base speed, fighting stab, water resistances, high synergy with skymin, good synergy with aegislash, the ability to 2hko most neutral switchins.
A lot of teams had trouble dealing with the high speed and coverage of keldeo with skymin. Keldeo-Skymin was a premiere lead duo that you had to base your lead against. You would be forced to lead something like scarf landorus-t, scarf kyurem-b, kanga, cresselia with an ice move, or something fast in order to disrupt the duo. Usually the opponent would be forced to concede at least 1 pokemon almost for free from the speedsters just killing it off first. Note: if one of the pokemon could not hit something super-effectively, the other one could usually hit it super-effectively; examples: amoonguss, heatran, rotom-w. And skymin could give the enemy spdef drops with seed flare to allow keldeo to punch through it.
How do/did you deal with this Pokemon in Doubles?
You put a skymin on your team and you called it a day. You could also use amoonguss, lati@s, and cresselia. Besides these guys, it was kinda hard to switch in on a keldeo since keldeo 2hko'd most neutrals. A common strategy against offensive teams is to allow 1 of your pokemon to die, and then switch in a faster pokemon to beat their pokemon (situation: let your heatran die to the keldeo so you can switch in your thundurus and kill it). This often works against offensive teams because of their lack of defensive switch-ins.
You could also use trick room, but keldeo offense teams would usually give you a beating while you're trying to set up the trick room, so it was a gamble (unless you're that guy running amoonguss, cresselia, abomasnow trick room).
Note: mega venasaur wasn't a thing back then, you used other megas like kanga.
Do you use Hydro Pump or Scald?
80% of people use hydro pump because it got kills on weakened charizard-y's and 2hko'd neutral switchins, but if you're a professional like me, you use scald because i'm NOT about missing. You can also forgo running a coverage move in your third slot to run: Hydro Pump/ Scald/ Secret Sword/ Protect. This has the benefit of juking your opponent the fuck out when they see the second water move. It has the downside of no quick guard or icy wind.
feel free to critique or add on to my keldeo analysis since i am old as dust.