Goliath was an earth pony, encountered alongside his mother as Blackjack was returning to Hoofington.
Goliath at some point, had an encounter with Killing Joke. The vile plant, magically altered him and turned him into a giant, possibly due to his name or perhaps because he would presumably starve to death, trying to feed himself at such a large size.
Goliath was encountered by Blackjack, pushing along train cars, loaded with flamer fuel and other flammable chemicals. The train cars were being pushed towards Fillydelphia, for use by Red Eye in cleansing Everfree Forest.
Goliath is a massive blue earth pony, several times larger than any other creature, excluding dragons.
He is likely immune to Killing Joke, having been already placed under one curse and he can use his superior size and strength to lift and move heavy objects, such as train cars.
He seems to be very obedient, especially towards his mother, who works out their financial issues and oversees his work, ensuring he has enough food.
His name is a reference to Goliath, the ancient giant who was killed with a sling.