The Werehog disease is inflicted on Sonic by Dr. Eggman and the Chaos Emeralds. This causes Sonic to turn into a "Werehog" every time the sun goes down. In day time, Sonic is normal and runs at blinding speeds, but when night comes he transforms into the Werehog which is slow and combat centered. The Werehog also has the ability to stretch his arms when appropriate.
Werehog is a misnomer, considering that "were-" refers to man. A werehog would be a man who took on porcine features.