Imagine the situation in which a small child participates in a game with teens or adults. Perhaps the child doesn't know or can't follow all the rules strictly. Or in sports, this child has physical limitations compared to the older, bigger, more coordinated participants. The child ends up playing a special role in the game. In Spanish this is called caballito blanco, palomita blanca, cascarón de huevo, etc., depending on the country and region. The person in this role is permitted to follow the rules of the game more loosely. The term can be used more generally, also, to refer to anyone who's given extra leeway for some reason or other.
What's the term for this in English?
Bonus question: How did the term originate?
This question was inspired by https://spanish.stackexchange.com/q/22950/9385.
I wonder if this would work:
My son sometimes used to beat me at Fish Eat Fish because he played so unpredictably. He was a total wild card.