There are geldings who are "proud cut." There's a small mass on the testicular cord, whatever you call it, above the testicles. Normally, the vet will cut above the massed tissue, which produces testosterone. In the case of a proud cut gelding, the vet will cut below the tissue and leave it there. Thus, you have a gelding with testosterone so they act like a stallion, but they cannot reproduce. We believe my pony may be proud cut, they poor guy tried mounting a TB mare!