Battle Royals/Royal Rumbles...
 

Has a surprise entrant ever won a Royal Rumble or Battle Royal?

Not a Royal Rumble. I don't know about Battle Royals though Tank Abott may have in WCW, I can't quite remember.

Yes. And I can't believe you don't know this cause it was recent. I never usually remember the recent stuff.

Oh, right, Angle won that SmackDown! battle royal...

Anyone else?

I'm trying to think. I don't have another one yet.

Angle is the only one I can think of, unless one happened in WCW that I cant recall.

Chyna won in the DX/Corporation Rumble

Yup

In the early days, AJ Styles won in a TNA X-Division battle-royal as a surprise participant.

Test won the "Don't Get Fired For a Year" battle royal at Survivor Series 2001. He was not originally part of the Battle Royal but he knocked out Scotty 2 Hotty backstage. When all of the wrestlers walked out for the Battle Royal, Test came out last technically as a surprise entrant.

At Wrestlemania 14, LOD won the tag battle royal, pretty sure they were a surprise.