Ekalavya
The young prince of Nishada, son of Hiranyadhanus, is a well known character in Mahabharata for the guru dakshina he gave - his right thumb!
It is said that after Dronacharya declined to take him as his student, Ekalavya learnt archery himself by making a clay idol of Dronacharya and accepting the idol as his Guru. When the Pandavas find a dog's mouth sewn up by arrows that neither harmed the dog nor let it bark, they and ultimately Dronacharya spot Ekalavya, and Dronacharya asks for his right thumb as Guru dakshina. Later, Ekalavya is killed by Lord Krishna in a battle.
There are two interesting perspectives for Ekalavya's guru bhakti. One, which glorifies Ekalavya, states that Dronacharya refused to accept him as a student as he belonged to lower caste and went on to get his right thumb as dakshina to make Arjuna, his dearest disciple the best archer in the world.
The other perspective, though, reasons out the logic and hidden purpose for Dronacharya's act. It claims that Ekalavya's mastery in archery would not serve good purpose as he belonged to the tribal kingdom Nishada who weren't law-abiding. Also, exhibiting his skills on a dog was not considered acceptable behaviour.
