Lord Brahma's vehicle is Hamsa, the Swan. He is also shown riding a chariot drawn by seven swans, representing the seven worlds.
Hamsa means ‘pure’. Lord Brahma uses Hamsa not only as his vehicle, but also to chant the Vedas. Lord Brahma himself is sometimes called Hamsa. Hamsa-yana, the airplane by which Brahma travels all over outer space, resembles a swan. Brahma is also known as Hamsa because he can grasp the essence of everything. Some references indicate that Brahma's swan carrier is akin to the vimanas (celestial spacecraft) run by mantric hymns, i.e., that Hamsa is a subtle mechanism of this kind, and not a sentient living entity.
Bhaktivedanta Book Trust. Excerpted from various sources, including text and Purport of HDG A.C. Bhaktivedanta Swami Srila Prabhupada.