Well mathematically speaking anything can be a black hole if u can compress it to a small enough size so as to make its density so big that not even light can escape its gravity you will have a black hole. The minimum size that you would need to compress an object into to make a black hole is called as its “Schwarzschild radius.”
If you could compress the size of Mount Everest into the order of Nano meter (almost the size of a HIV virus) you would’ve a black hole. That’s ridiculously dense!!!!!