  We use a robust analytical model together with a high-resolution
hydrodynamical cosmological simulation to demonstrate that in a $\Lambda$ cold
dark matter ($\Lambda$CDM) universe, a small fraction of dwarf galaxies
inhabiting dark matter (DM) halos in the mass range $3\times 10^{9} \lesssim
M_{200} / M_{\odot} \lesssim 10^{10}$ form unusually late ($z<3$) compared to
the bulk population of galaxies. These galaxies originate from the interplay
between the stochastic growth of DM halos and the existence of a time-dependent
DM halo mass below which galaxies do not form. The formation epoch of the
simulated late-forming galaxies traces remarkably well the time when their host
DM halos first exceeded a nontrivial (but well-understood) time-dependent
critical mass, thus making late-forming dwarfs attractive cosmological probes
with constraining power over the past growth history of their host halos. The
agreement between our model and the simulation results demonstrates that the
population of simulated late-forming dwarfs is a robust cosmological outcome
and largely independent of the specific galaxy formation model included in the
simulations provided: (1) the universe underwent cosmic reionization before
$z_{\rm re} \sim 8$; (2) star formation proceeds in gas that self-gravitates;
and (3) galaxy formation is largely restricted to atomic-cooling halos before
$z_{\rm re}$. The scarcity of massive late-forming dwarfs expected in
$\Lambda$CDM implies that the great majority of bright, metal-poor, and
actively star-forming dwarfs observed in our local universe--the most obvious
candidates for these late-forming galaxies--cannot be undergoing their
formation for the first time at the present day in a $\Lambda$CDM universe.
