Dark Horse

Dark horse is actaully a fairly common idiom that you can hear anywhere, but has more than one meaning. You heard me right...

I'd take being called a dark horse as a compliment, to be honest. Means you're capable of surprising people.

Meaning
1. Someone who might win a sports tournament and is - by rights - an unlikely candidate for this.

2. A mysterious character.

Use in Normal Speech
In sports

"I think she might be a dark horse for the tournament, what d'you reckon?"

After someone has made a surprising confession.

"You dark horse, Jen, I never had you down as a romantic!"

For more animal related idioms, look here.