rumored to be original travel form; stags are native to northern Kalimdor; some notable druids such as Malfurion Stormrage have stag antlers; Night Elf Moonkin Form has stag antlers (in contrast to the Tauren form which has horns)

sleek body, playful nature, prettier than the current tusked sea lion
Dolphin/Porpoise
can be summoned by druids in the RPG; characteristics similar to seals

from the Barrens and Mulgore; they are tall and strong like Tauren; known for their running ability
Zhevra
hooved animal of the Barrens, near the Tauren's home; larger and more robust than other 'runners' such as Gazelles; a rare spawn zhevra named Swiftmane can run at incredible speeds

from Northrend, they are a large maned mammal, and fitting since Tauren have a "lion" cat form

clearly a 'travel' beast; although not very exciting, fits Human culture well (since Worgen were Humans); there are Fel Steeds and Shadow Chargers in Shadowfang Keep
Worg
an obvious choice; the Northrend model looks pretty lean and fast

a fierce predator with sharp teeth; surely there are sharks in the waters surrounding Gilneas, as there are with its neighboring zones of Silverpine Forest and Hillsbrad Foothills

it's the troll racial mount (they are quick on their feet); found in Stranglethorn Vale and Echo Isles, where the Darkspear lived
Hyena
although not native to their homelands, Trolls may have encountered hyenas recently in the nearby Barrens, perhaps in their travels to learn Druidism from the Tauren; they seem to fit their personality well (that heckling laugh); I just think it would be awesome to have hyena as a travel form :)

tropical fish often share habitat with Trolls; their pointy-teeth smile and crazy blood-shot eyes almost scream Troll!