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.
1. Someone who might win a sports tournament and is - by rights - an unlikely candidate for this.
2. A mysterious character.
Use in Normal SpeechEdit
"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.