Lord Varaha

Lord Vishnu's third appearance was in the form of a boar. This avatar of Lord Vishnu rescued the earth which had been submerged in the ocean. He lifted it out the ocean with his tusk. A new creation started after that. A demon by the name of Hiranyaksha had abducted the earth and carried it to the 'Rasatala' (the second lowest planetary system in the universe ).

The lord entered that lower plane of existence and rescued Bhumi ( Mother Earth ) from that filthy place. Demons generally live in a unclean way and this is another reason the Lord appears as a boar.