Zoom!

Bust out a [npc=90205], watch it confuse the hell out of the snails, and watch Zoom cruise to an easy victory.

Reward: [item=137298] / [npc=107206] – the pet is automatically added to your journal upon completion. Blizzard hotfixed the pet in Patch 7.1, Zoom is not cageable anymore and you can have only one per account.

This achievement takes place in the Stonedark Grotto, a cave located in Highmountain, north of Bloodtotem camp and west to Torok's Bluff.

The snail race recurs every few minutes, you need to hold up the competitor snails to let Zoom win the race.
Beware, you can block not only other snails' but also Zoom's advance. Size-enlarging toys, like [item=118937] or [item=109183] can help on this task.

This was my way to complete the achievement:

► Zoom! – YT video guide

There's another method, found by Megrathea, using [item=98136] to eat up competitor snails. This is an easier way – if you happen to have that toy. This toy is a rare drop from [npc=68220]s in the Throne of Thunder raid instance.

There are probably other "workarounds" too.

Came across a far easier strategy while questing and got the achievement accidentally though it involves a rather rare toy..

Used [item=98136] and ran Jerry over the other snails but stayed clear from Zoom. Jerry ate them and they dissapeared leaving Zoom the only one to finish the race at his own snail pace. Upon crossing the finish line got both achievement and pet.