They are called RIVER DOGS for a reason. It probably is seeing the aft part of its body and thinks it another fish...not to bright. Very aggresive fish and it will probably get worse, or it will change and focus its aggression. Whatever the case, prob be a good idea to secure the tops on your aquarium.
My oscar is 13" and growing....growing into a major pain in the a!! Spashing water, attacking the glass, chasing tank mates, moving rocks(twice his size), ripping up plants, pouting and begging for food. Yep, thats the 'ol River Dog for ya!! Fun.
LOTS and LOTS of water changes...dirty fish!!