Big bones are the fourth best bones in the game. They give 1050 experience for a Regular player and they give 150 experience for a Legend or Iron man. The experience goes towards the Prayer skill.
There are multiple monsters which drop Big bones, listed under here.
Monster Combat level Quantity Rarity 
Giant roc 192 1 Always 
Tortoise 92 1 Always 
Wingman skree 143 1 Always 
Flockleader geerin 149 1 Always
Flight kilisa 159 1 Always
Chaos elemental 305 5 Common
Kree'arra 580 1 Always
Moss giant 84 1 Always
Fire giant 86 1 Always
Ice giant 53 1 Always
Hill giant 28 1 Always
General graardor 824 1 Always
Cave horror 80 1 Always
Jungle demon 195 1 Always
