The most different traits in ghouls would most definitely be their element and the power they have with the element. They don't really have different bodily differences. Any ghoul can have any type and style of horns, or height.
Fire:
Fire ghouls elemental difference would be they can withstand certain amount of heat, and of course have a much hotter temper. You can distinctively tell a Fire ghoul by their red eyes. Fire ghoul's forms can be different, but mainly just more menacing, from having lava as their spit, or their hair and horns on fire, to even putting there hands to fire with glowing bright orange eyes.
Water:
Though many think Water's have different bodily parts, they don't, other then in Water. When they are submerged in a large body of water, it can bring out some of their more interesting features such as: The lower half of their body becoming a fish, therefore, Merghouls. Tentacles growing from their backs and growing gills on their necks and ribs and webbed fingers, but only in deep depths of water can they summon the different bodily parts.
Quintessence:
Quintessence ghouls don't have a special part to their element. They are the ghoul's of life, therefore can shift to whatever they wish, but their main unglamoured is a werewolf type.
Earth:
Earth's have a very special form with their element. They can shift to any animal they desire, except for creatures like unicorns, dragons, manticores etc. They can only shift to the earthly animals. Earth's are well known for their height and green eyes. Some earth's do have antlers for horns as well.
Multi:
They can't fully shift, but they can add more bodily parts and reshape them. They can have wings, tentacles, longer fangs, make themselves look more demonly. The only way to tell a multi is there yellow eyes.
Air:
Air's special form is any creature with wings. From pegasi to griffins, they can shape into any form, including growing wings of there own. The way to tell Air's apart is their eye color typically is white, grey, pink or a hazel.
