Why does An-225 have anhedral wings though it is a cargo aircraft? What are the rationale behind this anhedral high mounted wing configuration? Is that usual for very large carge aircraft?
Anhedral wings will induce roll instability and improve roll maneuverability. 

In a large/heavy airplane with a high-wing configuration there is usually excess roll stability, so this type of wings can be pretty common.

Both the high wing configuration and wing sweep contribute a negative sideslip-induced rolling moment, and anhedral is necessary to limit this moment. If sideslip causes strong rolling, the aircraft will have a tendency to dutch roll.