When Apex Altisaur enters the battlefield, it fights up to one target creature you don't control.
Enrage—Whenever Apex Altisaur is dealt damage, it fights up to one target creature you don't control.
It brings belligerence to new heights.
illus. Simon Dominic # 31/302
Quote from Raikoben »"Up to one target" means you can choose 0 targets and stop the interactions.