In rune scape what monster drops shark

I don't think that any monster drops sharks. Food in general is difficult to get from drops; you usually get food by fishing, killing monsters, etc.

Hilmarz is incorrect here. A couple known Monsters in Runescape that drop Sharks are Waterfiends (drop raw Shark 3-6), Aquanites, Ice Troll Runts (noted 1-6), Mogres, and Brine Rats (3-8 noted). Those are just the ones I remember from training on them, and I am quite sure there are others as well. Any good fansite Bestiary will help you in this area.