Yes there are. When the core density of a massive neutron star exceeds 10^15 grams/cc, there is, according to some calculations, a transition of the core material into a quark-gluon plasma. The neutrons loose their identity in the central 20 percent or so of the neutron star, and dissolve into quarks. Since quarks are fermions, they too are capable of producing a fermi degeneracy pressure like electrons do in white dwarfs, or neutrons do in most neutron stars. My understanding is that these objects are stable, but probably rare.