yes, they are called imaginary numbers.
No, we cannot actually find the square root of a negative number... since no number squared is a negative number. But, yes, we have invented an abstract artifact, so to speak, that allows us to describe such roots. The constant, i, is arbitrarily assigned to describe the phenomenon, but it holds no real numeric meaning. We simply have real multiples of 'i', which are equally meaningless, but artificially create a unique number line of imaginary values... when we square it, then we have a real workable number again.
Do square roots actually exist? Difficult to interpret how you mean that. No, you cannot have a quantity of objects that can be counted in the imaginary numbers. But yes, real, physical phenomenon can have imaginary values... technology, particularly complex physics and electronics, wouldnt even be possible without the advent of imaginary numbers. They have practical, tangible use.
Consider this... are the 'real' counting numbers real? The numbers you have been taught since kindergarten are no more real... they are abstractions of the mind. No 'number' physically exists, be it imaginary or real.
Thus, the quotation below.