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 Vajirapāṇi’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 Ambaṭṭha, DN.i.95 and again in that of Saccaka Nigaṇṭhaputta. MN.i.231