  The WIMPzilla hypothesis is that the dark matter is a
super-weakly-interacting and super-heavy particle. Conventionally, the
WIMPzilla abundance is set by gravitational particle production during or at
the end of inflation. In this study we allow the WIMPzilla to interact directly
with Standard Model fields through the Higgs portal, and we calculate the
thermal production (freeze-in) of WIMPzilla dark matter from the annihilation
of Higgs boson pairs in the plasma. The two particle-physics model parameters
are the WIMPzilla mass and the Higgs-WIMPzilla coupling. The two cosmological
parameters are the reheating temperature and the expansion rate of the universe
at the end of inflation. We delineate the regions of parameters space where
either gravitational or thermal production is dominant, and within those
regions we identify the parameters that predict the observed dark matter relic
abundance. Allowing for thermal production opens up the parameter space, even
for Planck-suppressed Higgs-WIMPzilla interactions.
