Where is the mini-boss in lord jabu-jabu in Zelda ocarina of time

Ahoy, there!
If you mean Bigocto, he is where the Spiritual Stone is being placed. You fight him after Princess Ruto is taken away. To reach here, you need to have cut the tail of the green thing that is in the first room you meet Ruto. All you have to do is jump (with Ruto) in the hole the green thing was blocking.