The hill christened "Slime Hill" by the people who find it, it is the hill outside Angel Falls that is inaccessible until post-game. It is named so because, as you would guess, the only monster family (not monster) that appears there is the Slime Family.
This is not nearly as bad as it sounds, as all these slimes appear at Slime Hill:
NOTE: It may be possible for slimes to form into slime stacks or king slimes, but if so, it is not likely.
Also, in the cave on Slime Hill is a boy who can give a quest, and he needs a wakerobin to activate the wishing water that is in the cave.