Purusha Mruga was renowned as one of the biggest devotees of Lord Shiva. As the name suggests, he was half man and half animal. In the Mahabharata, the Pandavas were performing the Rajasuya Yagna. Yudhishthira suggested that Purusha Mruga's presence in the Rajasuya Yagna would be most auspicious and assigned Bhima to invite the revered guest. Bhima, on his way, is said to have met an old monkey who was Hanuman in disguise. After inquiring about the purpose of Bhima's journey, Hanuman plucked three strands of hair from his tail and handed them to him, saying it would be of help later. Taking blessings of Hanuman, Bhima continued on the journey. On meeting Purusha Mruga, Bhima introduced himself and requested the kind devotee's presence in the Rajasuya Yagna. Purusha Mruga calmly stated that he would attend the ceremony only on one condition. Bhima would have to run towards Hastinapur and he would try to catch him. In the event that Bhima was caught outside the boundary of Hastinapur, Purusha Mruga would eat him. Since Yudhishthira was very keen on Purusha Mruga's attendance, Bhima accepted the challenge.
Bhima ran very fast and was confident that he could not be caught. He turned back, though, only to find Purusha Mruga gaining on him and almost about to catch him. Bhima remembered about the three hair strands which Hanuman gave him. He dropped one of them and found it was replaced by millions of Shiva Lingam. Purusha Mruga, being a Shiva Bhakta, stopped to pay his respects to each of these. Bhima, pleased by the boon, nevertheless continued running, but soon he found Purusha Mruga trailing him closely after worshipping all the Shiva Lingams. He dropped the second hair. The same process ensued. Similarly, Bhima dropped the third hair too. Finally, when nearing the door of Hastinapur and almost crossing it, Purusha Mruga got hold of Bhima's leg. Purusha Mruga told Bhima that he would eat him. Bhima, though, claimed to have entered Hastinapur. At that moment, Yushishthira and Krishna arrived on the scene. After narrating the entire incident to Yudhishthira, Purusha Mruga demanded justice. Yudhishthira pointed out that Bhima's leg was the only part outside the boundary of Hastinapur and therefore, legally Purusha Mruga could eat his leg only. Pleased by the impartial and thoughtful justice imparted by Yushishthira (Dharmaraja), Purusha Mruga showered lavish praise on him and let Bhima go. He blessed the Rajasuya Yagna and the Pandavas by attending the ceremony.
This is a later folk tale glorifying a local devotee or saint. Pandavas never performed Rajsuya Yagna at Hastinapur. They were then established at Indraprastha.