#3 Finn Balor
In a way, it is hard to call the first-ever WWE Universal Champion Finn Balor a dark horse.
Balor has recently aligned himself with The Club and this should give him a significant run in the Rumble with the help of his 'Good Brothers'.
A female WWE star said her life is in danger. Details HERE
Also, the viewers love a good conspiracy theory and it could be that the story of Vince not believing Balor to be "over" was actually planted to help him gain some support. After all, the WWE fans love to support an underdog. Especially if the 'Authority' seems to be holding the superstar down.
Perhaps Balor's showdown against Brock Lesnar for the Universal Championship was never really cancelled but delayed for a bigger stage.