  Polar ring galaxies, where matter is in equilibrium in perpendicular orbits
around spiral galaxies, are ideal objects to probe the 3D shapes of dark matter
halos. The conditions to constrain the halos are that the perpendicular system
does not strongly perturb the host galaxy, or that it is possible to derive
back its initial shape, knowing the formation scenario of the polar ring. The
formation mechanisms are reviewed: mergers, tidal accretion, or gas accretion
from cosmic filaments. The Tully-Fisher diagram for polar rings reveals that
the velocity in the polar plane is higher than in the host plane, which can
only be explained if the dark matter is oblate and flattened along the polar
plane. Only a few individual systems have been studied in details, and 3D
shapes of their haloes determined by several methods. The high frequency of
warps could be explained by spontaneous bending instability, if the disks are
sufficiently self-gravitating, which can put constraints on the dark matter
flattening.
