You can use name tags, but it seems that the mobile version 0.12.0 does not have name tags)
First place the name tag (can be used in Hell Fortress, Monster Spawner, Abandoned Mine, Village (obtained from the Blacksmith Shop, Desert Temple, and Jungle Temple) and change the name on the anvil (this is the name you want to name the creature; it costs level 5 experience)
Then aim at the creature you want to name. Just right click
Hope to adopt*-*