Found the following entry in wikipedia, while searching on Hanuman on Arjuna's Flag in google. But haven't heard this version of story anywhere though ... Also that Ghatokacha had a son !!
" In the MahabharataHanuman is also considered to be the brother of Bhima, on the basis of their having the same father, Vayu. During the Pandavas' exile, he appears disguised as a weak and aged monkey to Bhima in order to subdue his arrogance. Bhima enters a field where Hanuman is lying with his tail blocking the way. Bhima, unaware of his identity, tells him to move it out of the way. Hanuman, incognito, refuses. Bhima then tries to move the tail himself but he is unable, despite his great strength. Realising he is no ordinary monkey, he inquires as to Hanuman's identity, which is then revealed. At Bhima's request, Hanuman is also said to have enlarged himself to demonstrate the proportions he had assumed in his crossing of the sea as he journeyed to Lanka and also said that when the war will come, I will be there to protect the Pandavas.During the great battle of Kurukshetra, Arjuna entered the battlefield with a flag displaying Hanuman on his chariot.[14] The incident that led to this was an earlier encounter between Hanuman and Arjuna, wherein Hanuman appeared as a small talking monkey before Arjuna at Rameshwaram, where Rama had built the great bridge to cross over to Lanka to rescue Sita. Upon Arjuna's wondering aloud at Rama's taking the help of monkeys rather than building a bridge of arrows, Hanuman challenged him to build a bridge capable of bearing him alone; Arjuna, unaware of the vanara's true identity, accepted. Hanuman then proceeded to repeatedly destroy the bridges made by Arjuna, who decided to take his own life. Vishnu then appeared before them both after originally coming in the form of a tortoise, chiding Arjuna for his vanity and Hanuman for making Arjuna feel incompetent. As an act of penitence, Hanuman decided to help Arjuna by stabilizing and strengthening his chariot during the imminent great battle. After, the battle of Kurukshetra was over, Krishna asked Arjuna, that today you step down the chariot before me. After Arjuna got down, Krishna followed him, then Krishna, thanked Hanuman to stay with them during the whole fight in form of flag on the chariot. Hanuman came in his original form, bowed to Krishna and left the flag, flying away into the sky. As soon as he left the flag, the chariot began to burn and turned into ashes. Arjuna was shocked to see this, then Krishna told Arjuna, that chariot, was standing apiece only due to powers of mine and Hanuman, otherwise, it would have burnt many days ago due to effects of celestial weapons thrown at it in the war.According to legend, Hanuman is one of the four people to have heard the Bhagwad Gita from Krishna and seen the Vishvarupa form, the other three being Arjuna, Sanjaya and Barbarikason of Ghatotkacha.