
Thornback Cowfish - Lactoria fornasini
Lactoria fornasini are called Cowfish for the 2 'horns they have on top of their head and Thornback for the 'thorn/spine' on its back. Am not sure, but the blue markings probably is its sign to indicate that it is poisonous, if eaten.
They are small in size, usually around 8-10 cm.

"Lactioria fornasini", the thornback cowfish, is a poisonous species of boxfish found throughout the Indo-Pacific from East Africa to Rapa Island. It can grow to 23 cm in length.
Similar species: Box, Puffer And Trigger Fishes
By Albert Kang
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Uploaded Aug 13, 2016. Captured Jul 4, 2016 14:11 in Unnamed Road, Daanbantayan, Cebu, Philippines.