Does negative frequency actually exist or it is just theoretical? This below questions gives good explanation regarding negative frequency 

What is the physical significance of negative frequencies?

But I have one query


Does negative frequency only exist theoretically or does it actually exist in nature and can be measured?
Consider a wheel rotating counter-clockwise at one revolution per second. Its frequency of rotation is 1 Hz. If it rotates clockwise, its frequency of rotation is -1 Hz. It's as simple as that.