Hanuman in Mahabharata - Did you know this?

During the Pandavas' exile, he appeared disguised as a weak and aged monkey before his (spiritual) half-brother, the Pandava prince Bhima in order to subdue his arrogance and teach him the value of humility.

More significantly, during the great battle of Kurukshetra, Arjuna entered the battlefield with the flag of Hanuman on his chariot. The incident that led to this was an earlier ecounter between Hanuman and Arjuna; Hanuman appeared as a small talking monkey before Arjuna at Rameshwaram, where Sri Rama had built the great bridge to cross over to Lanka to rescue Sita. Upon Arjuna's wondering out aloud at Sri Rama's taking the help of "monkeys" rather than building a bridge of arrows, Hanuman (in the form of the little monkey) challenged him to build one capable of bearing him alone, and Arjuna, unaware of the monkey's true identity accepted. Hanuman then proceeded to repeatedly destroy the bridges made by Arjuna who became depressed and suicidal, and decided to take his own life. Vishnu then appeared before them both, chiding Arjuna for his vanity, and Hanuman for making the accomplished warrior Arjuna feel incompetent. As an act of 'penitence', Hanuman agreed to help Arjuna by stabilising and strengthening his chariot during the then-likely great battle.

Hanuman, thus, had the fortune of hearing the Gita as recited by Sri Krishna himself. Many worshippers of Lord Krishna (who drove Arjuna's chariot) keep flags of Hanuman as a mark of respect to his great devotion, and honour Hanuman. For instance, the Sri Krishna Matha at Udipi bears an idol of Hanuman (known as Mukhyaprana at the site), and all offerings made to Lord Krishna are also offered to Hanuman before being distributed to devotees.