The animal that hangs out in melon patches is the elusive fox! They like to steal melons then come back into town disguised as human melon sellers. Foxes are tricky like that.
If you want to stop foxes from stealing your melons you can get one of those kits that neigh (きつね) when one is around. The kits neigh when they sense a fox, but because it sounds like a horse the fox won't run. But you'll be able to get out there and take your melons back!