Blue Unicorn

Found in schools, patrolling mainly along outer reef walls, in pursuit of plankton.
Western Central Pacific - Indonesia, Philippines

Surgeonfish have a blade like spine in the tail that points outwards when bent, unicorns have two hook-like plates along the tail, these are used for defence and are as sharp as a surgeons scalpel, hence the name "Surgeonfish".
Some species are venomous.