This is an official character from The Lion King: Six New Adventures books. This character is not seen in any of the movies.
Boma is a buffalo who lived under the reign of Ahadi and then Mufasa.
Boma first appears in "A Tale of Two Brothers" when he and his herd are occupying the only water source during a terrible drought in Prideland. Taka tricks his brother Mufasa to come to the water hole. When Mufasa begins reasoning with Boma, Taka roars and says that Boma must move by order of the Lion King, or they will have to face in combat. When Mufasa escapes, Boma's herd attack Taka, leaving him a well-recognizable scar on his left eye.
Boma is later seen in "How True, Zazu?" when he helps to stop the invasion of army ants.
Boma appears in "A Tale of Two Brothers" and "How True, Zazu?".