The
SRD says "You can move through a square occupied by a helpless opponent without penalty. (Some creatures, particularly very large ones, may present an obstacle even when helpless. In such cases, each square you move through counts as 2 squares.)"
From that I only have Huge or greater sized creatures make difficult terrain when they are defeated.
The
SRD also says about Crowds "It takes 2 squares of movement to enter a square with crowds."
The DMG II, pg. 59 says "As a general rule, a single 5-foot square ocupied by a crowd of Medium creatures contain three Medium creatures."
From all of this, I would extrapolate that a square would need at least three Medium corpses to make it difficult terrain.