Not to be teased, not to be touched, not to be trusted. The Wurm is a relative of he dragon- minus arms, legs, and wings. Basically, take a snake and attach a lizard's head (with horns!). Now increase size to 100 yards length. There! you have a Wurm.
Dangerous and Fickle, Wurms are, in Magic: The Gathering green's answer to red's dragons. Massive creatures, mainly with Trample, these beasts tend to ignore what the opponent has and attack. They vary in size, from the 4/4 trample Arrogant Wurm to the 7/6 Scaled Wurm, and even the 7/7 trample Shivan Wurm. Dangerous and always huge, Wurms are a newbie's favourite creatures... and sometimes, an experts favourite, too.
Though Wurms often have disadvantages, they are still deadly creatures; Some can be complex number games, others vanilla. But they are all BIG.
In other fantasy games, they follow the same pattern: Almost unkillable, massive, and omnivorous, they devour anything in their region (normally forests, sometimes mountains and the sea*). Wurms tend not to be sapient, but in some games they are; They are always frightening. Though related to the Dragon, they lack a breath weapon, and claws; They kill in two ways. One: Crushing. Two: Biting.
(we can assume that, in magic at least, those who crush tend to have trample...)
* IE, habitat. Though some may eat such.