  It has been argued that black hole solutions become unavoidably
time-dependent when the graviton has a mass. In this work we show that, if the
apparent horizon of the black hole is a null surface with respect to a fiducial
Minkowski reference metric, then the location of the horizon is necessarily
time-independent, despite the dynamical metric possessing no time-like Killing
vector. This result is non-perturbative and model-independent. We derive a
second law of black hole mechanics for these black holes and determine their
surface gravity. An additional assumption establishes a zeroth and a first law
of black hole mechanics. We apply these results to the specific model of dRGT
ghost-free massive gravity and show that consistent solutions exist which obey
the required assumptions. We determine the time-dependent scalar curvature at
the horizon of these black holes.
