  We present new evidence for eight early-type galaxies (ETGs) from the CALIFA
Survey that show clear rotation around their major photometric axis ("prolate
rotation"). These are LSBCF560-04, NGC 0647, NGC 0810, NGC 2484, NGC 4874, NGC
5216, NGC 6173 and NGC 6338. Including NGC 5485, a known case of an ETG with
stellar prolate rotation, as well as UGC 10695, a further possible candidate
for prolate rotation, we report ten CALIFA galaxies in total that show evidence
for such a feature in their stellar kinematics. Prolate rotators correspond to
~9% of the volume-corrected sample of CALIFA ETGs, a fraction much higher than
previously reported. We find that prolate rotation is more common among the
most massive ETGs. We investigate the implications of these findings by
studying N-body merger simulations, and show that a prolate ETG with rotation
around its major axis could be the result of a major polar merger, with the
amplitude of prolate rotation depending on the initial bulge-to-total stellar
mass ratio of its progenitor galaxies. Additionally, we find that prolate ETGs
resulting from this formation scenario show a correlation between their stellar
line-of-sight velocity and higher order moment h_3, opposite to typical oblate
ETGs, as well as a double peak of their stellar velocity dispersion along their
minor axis. Finally, we investigate the origin of prolate rotation in polar
galaxy merger remnants. Our findings suggest that prolate rotation in massive
ETGs might be more common than previously expected, and can help towards a
better understanding of their dynamical structure and formation origin.
