A style guide is a document of text and reference illustrations used to guide artists who work on a Magic set. The concept illustrations shown in this Arcana are all by Guildpact's lead conceptual artist, D. Alexander Gregory.
When the Gruul raid the territories of the other guilds, they employ ferocious Gruul Nodorog and Rumbling Slum like a combination pack animal and Battering Wurm. As you can see below, some Gruul clansmen specialize in handling these raiding beasts.
The Selesnya Conclave, as you'll remember, employs giant beasts such as Siege Wurm as well. But the look of a Gruul wurm is much more savage and primal -- you'll find no tamed animals wearing chrome armor in this guild.
Wurms in particular are employed by the Gruul as a kind of living explosive device. Check out Chris Moeller's art on Killer Instinct, depicting a wild wurm-outa-nowhere.