  In this paper, we show that the Jacobiator $J$ of a pre-Courant algebroid is
closed naturally. The corresponding equivalence class $[J^\flat]$ is defined as
the Pontryagin class, which is the obstruction of a pre-Courant algebroid to be
deformed into a Courant algebroid. We construct a Leibniz 2-algebra and a Lie
2-algebra associated to a pre-Courant algebroid and prove that these algebraic
structures are isomorphic under deformations. Finally, we introduce the twisted
action of a Lie algebra on a manifold to give more examples of pre-Courant
algebroids, which include the Cartan geometry.
