Wherever a Magcargo slithers, its trail of flames is not far behind. Insanely hot body temperatures heat the ground up like a volcanic eruption, and their slow, legless method of locomotion leaves their own lava there long enough to sink into the ground, burning through the Earth's crust and creating a river of lava prone to bursting into flames.
This is one of the reasons that, despite their obvious power, many trainers decline to use Magcargo: even though they spend the vast majority of their time carried in pokeballs, few trainers wish to clean up after them after they are sent into battle. Water is often used to combat fire, but against this heat it will simply evaporate; instead, one must wait hours for the trail to begin to cool, then cover it with enough rocks and dirt to stop its flow. It is not an accident that Blaine of Cinnabar Island, their most notorious trainer, holds his battles inside an active volcano where his Magcargo will simply be another source of magma.
Ironically enough, Magcargo were once prized for this very ability. Moats were usually made of water, but wealthy lords or those living on volcanic islands often preferred Magcargo lava as a defensive material, for water is not nearly as terrifying. Although few ever were so daring as to lay siege to such castles, they contained large food stockpiles all the same. This was because at times the Magcargo flames would burn away the bridge connecting the castle to the outside world, leaving the people there isolated for months until the moat began to cool.