A Yakkha.
It is said that whoever, even up to the third time of being asked, refuses to answer a reasonable question put by a Buddha, his head will split into pieces on the spot. It was Vajirapāni's duty to frighten such people by appearing before them in the sky, armed with a thunderbolt, which he was ready to hurl if necessary. He was visible only to the Buddha and the person in question.
Two instances of this are given in the books - once in the case of Ambattha, (D.i.95; cf. the story in the Ayakūta Jātaka) and again in that of Saccaka Niganthaputta (M.i.231).
Buddhaghosa says (DA.i.264; MA.i.457; cp. Dvy.130) that Vajirapāni is identical with Sakka, and proceeds to describe the fierce appearance assumed by him on these occasions. This arrangement was made in fulfilment of a promise made by Sakka, in the presence of Mahā Brahmā, when the Buddha was reluctant to preach the Dhamma (See Vin.i.5f), that if the Buddha would establish his rule of the Dhamma (Dhammacakka), Sakka would afford it the necessary protection.
In some places (E.g., Cv.xcvi.37; see also J.R.A.S. 1916, p.733f), Vajirapāni's conquest of the Asuras is alluded to, thus establishing his identity with Indra. See also Vajirahattha.