  Magnetars are potential energy sources or central engines for numerous
transient phenomena in the Universe. How newborn magnetars evolve in different
environments remains an open question. Based on both observed and candidate
magnetars, it is found that the periods of all magnetars or candidates appear
as a bimodal distribution, and are defined as the ``long-P'' and ``short-P''
magnetar subclasses, respectively. We find that for the ``short-P'' subclass of
magnetars, the $\dot{P}$ values also appear as a bimodal distribution, and
therefore can be classified as ``high-$\dot{P}$ short-P'' and ``low-$\dot{P}$
short-P'' magnetar subclasses. In this paper, we use Monte Carlo simulations to
generate synthetic magnetar populations and investigate the evolution of the
``high-$\dot{P}$ short-P'' and ``low-$\dot{P}$ short-P'' magnetar subclasses by
considering both the magnetar spin and inclination, as well as the decay of
their magnetic field within their evolution in both vacuum and plasma-filled
magnetospheres. We find that the magnetar evolution is dependent on both spin
and magnetic field, but seems to be insensitive to inclination evolution and
magnetospheric environment for the ``high-$\dot{P}$ short-P'' sub-class. In
comparison for the case of ``high-$\dot{P}$ short-P'', the magnetar evolution
is dependent on spin, magnetic field, and inclination evolution, as well as the
magnetospheric environment. The best evolution model should be the case of
inclination evolution in vacuum with a small value of
$\overline{\mathrm{FOM}}$. The differences in the best-fit parameters also
suggest that the ``high-$\dot{P}$ short-P'' and ``low-$\dot{P}$ short-P''
magnetar subclasses may be tracking with different evolution channels.
