In this nice paper the author proves that all natural symplectic forms on the tangent bundle of a pseudo-Riemannian manifold are pull-backs of the canonical symplectic form on the cotangent bundle with respect to some diffeomorphisms which are naturally induced by the metric.
@article{701583, title = {Natural symplectic structures on the tangent bundle of a space-time}, booktitle = {Proceedings of the 15th Winter School "Geometry and Physics"}, series = {GDML\_Books}, publisher = {Circolo Matematico di Palermo}, address = {Palermo}, year = {1996}, pages = {[153]-171}, mrnumber = {MR1463517}, zbl = {0908.53017}, url = {http://dml.mathdoc.fr/item/701583} }
Janyška, Josef. Natural symplectic structures on the tangent bundle of a space-time, dans Proceedings of the 15th Winter School "Geometry and Physics", GDML_Books, (1996), pp. [153]-171. http://gdmltest.u-ga.fr/item/701583/