My battle pet, Carnage, beat Angry Arnold 5 times and got the armored egg which I gave to him. I decide to do a bit of battling with my other pets so they can get better jobs. My pet, Drusilla, has now beaten Angry Arnold 5 times and got an armored egg.

Is it cheating if I use the egg on Carnage or must I use eggs on the pets which won them?

You're free to use them on any pet you please; but you can't use them on any pet above tier 3. Almost all of the easier permanent challengers, up to Tide, will give you eggs to use on your pet.

Thanks. I'll use them on Carnage, then, because I won't battle with the other pets much.