. . . humans from the Harry Potter universe through some magical, inter dimensional spell ended up in Equestria. As a rule of thumb, I will assume witches and wizards would mostly turn into unicorns with a few transformed into Alicorns. Muggle (non-magical) humans would turn into Earth ponies or pegasi.
Albus Dumbledore probably would be an Alicorn, while Harry Potter, and members of the Order of the Phoenix and Dumbledore's Army would be unicorns.
Voldemort would be an Alicorn while his Death Eaters would be unicorns.
The Mane Six would of course ally with Dumbledore and Potter and try to use the Elements of Harmony.
Lord Tirek MIGHT ally with Lord Voldemort and the Death Eaters for awhile, but Voldey or Tirek would sooner or later turn on the other.
Discord, being traditionally a chaos-oriented dracoequus, would be the ultimate wild card.