  RX J0720.4-3125 is the third brightest neutron star in the soft X-ray sky and
has been a source of mystery since its discovery, as its long 8-s period
separates it from the population of typical radio pulsars. Three models were
proposed for this source: a neutron star accreting from the interstellar
medium, an off-beam radio pulsar, or an old, cooling magnetar. Using data from
Chandra, ROSAT, and BeppoSAX we are able to place an upper limit to the period
derivative, $|\dot{P}| < 3.6\times10^{-13}{s s}^{-1}$ (3-$\sigma$). While our
upper limit on $\dot P$ allows for the accretion model, this model is
increasingly untenable for another similar but better studied neutron star, RX
J1856.5-3754, and we therefore consider the accretion model unlikely for RX
J0720.4-3125. We constrain the initial magnetic field of RX J0720.4-3125 to be
$\lsim 10^{14}$ G based on cooling models, suggesting that it is not and never
was a magnetar, but is instead middle-aged neutron star. We propose that it is
either a long-period high-magnetic field pulsar with $\dot P\sim 10^{-13}{s
s}^{-1}$ similar to PSR J1814-1744, or a neutron star born with an initial
period of $\approx 8.3$ s and $\dot P\sim 10^{-15}{s s}^{-1}$. The proximity of
RX J0720.4-3125 is strongly suggestive of a large population of such objects;
if so, radio pulsar surveys must have missed many of these sources.
