Choose target creature you control and target creature you don't control. If the creature you control entered the battlefield this turn, put a +1/+1 counter on it. Then those creatures fight each other.
If the target creature you control entered the battlefield this turn and is still a legal target as Malamet Battle Glyph resolves but the target creature you don't control isn't, you will still put a +1/+1 counter on the creature you control. The creatures will not fight each other.