[MunchkinMan]

I'm looking for Fairy Dragon now to check the text, but we've ruled that unless the card which allows Monster removal specifies a time frame, you can choose to use a Monster removal action at any point during the combat.

[Edit] After looking over the card, I would say it's legitimate to take advantage of this ability at any point during the combat, even if you'd initially thought to fight it. Obviously, once you've decided to have it come to your side, you can't reverse that.