In Hindu mythology, the elephant headed god, Lord Ganesha, is also called “Endantha” which literally translates to “One Tooth”.
There is an interesting story to this.
Once upon a time, Ganesha had two tusks. Parasuraman promised to fulfill the wish of his mother Renuka Devi, to take revenge on the entire clan of Kshatriya, along with the King Karthyaveeraarjuna, who killed his father Jamadagni. For this sake, he did penance to get weapons from Lord Shiva. He gave the weapons to Parasuraman, which were Parashu and Pasupathaasthra. Parasuraman killed the entire clan of Kshatriya for 21 times, and came to Kailasa to take the Darshan of Lord hSiva, after doing another penance for him.
Ganesha refused to let him in to not disturb Shiva and Parvathi. Then Parasuraman tried to hit him with Parasu. Ganesh lifted him with his trunk and harassed him very much. He lifted Parasuraman upto Goloka where he had Lord Krishna darshan. Ganesha then soothed him and gave respect to the weapon Parasu, which was given by his father. He gave one of his two tusks to the axe, “Gajadantha” for not wasting the effect of the weapon.
From then onwards, Lord Ganesh became “Ekadantha”.
This is all from the scripture “Brahma Vaivartha Purana”.