The buffalo is the vehicle or messenger of the Dharmaraja Yama, the Lord of Death. Dharmaraja was subdued by Manjushri in the form of Yamantaka, becoming a protector of dharma practitioners, the name dharmaraja means King of the Dharma. This then is used to take the energy and power of hatred (of things negative or bad) and use this energy to achieve a beneficial purpose. HH explains it this way: With compassion as the causal motivation... the practitioner utilizes hatred or wrath for a specific purpose. This technique is based on the fact that when we become angry, a very energetic and powerful mind is generated. When trying to achieve a fierce activity for beneficial purposes, the energy and power make a difference. Thus it is because of the usage of hatred in the path in this way that there come to be wrathful deities.