My fraternity (Phi Kappa Sigma) has been going by that name since we started in 1850 (mainly, 'Phi Kaps' and 'Skulls'.) My Skull Sessions (our new member handbook) lists the following "Phi Kappa x" fraternities as:
Phi Kappa Sigma, 1850
Phi Kappa Psi, 1852
Phi Kappa Theta, founded 1899
Phi Kappa Tau, 1906
I verified that on the NIC's website, as well as each fraternity's website. On my campus (Georgia Tech) along with most other campuses I have been to, Phi Kappa Theta goes by the name "Phi Theta", Phi Kappa Psi goes by "Phi Psi" and Phi Kappa Tau goes by "Phi Tau." I'm sure there's differences here and there at pretty much every campus, but usually I hear "Phi Kaps" or "Skullhouse" attached to Phi Kappa Sigma when they're on that campus.
Regardless, we can claim we're the original Phi Kaps
