In the book Prapannamrita, we hear the story of Sri Tiruppana, who was born a low caste chandala in South India.
Once, he lost consciousness while chanting the Holy Names of Sri Hari by the bank of the river Kaveri. At that time, a brahmin priest named Muni was returning to the Sri Ranganath temple with water for the Deity’s abhishek.
Seeing the Sri Tiruppana lying on the bank of the river, he repeatedly called out to him in a rude manner. Disdaining to touch the lowborn person with his hands, and fearful that the water meant for the Deity might become impure, he threw a lump of earth at the prostrate figure and awakened him.
Arriving at the temple, Muni found that the doors were bolted from inside.
After calling out for a long time, the priest heard a voice speak to him. The voice said that Sri Ranganath Himself had been injured by the lump of earth that had been thrown at His humble servant by this proud priest. Unless the priest circumambulated the temple carrying the devotee on his shoulders, the doors of the temple will not open.
Thus, it came to pass that the priest did as he had been ordered, and only then the doors of the temple open.
The Tiruppani Sri Vaishnavas still worship Tiruppana as Muni-vahana because a bhramin named Muni had carried him (acted as his vahana) around the temple.
Source: Pearls of Devotion