Wanted to share an Exotic Mountseller theorycraft I've been having fun with in Casual. 

Basic synergies:

• Mountseller + 1-cost spells = spicy board
• 1-cost spells (i.e. Arcane Shot & Rapid Fire) + Vereesa Windrunner = finishing damage (or board control)
• Arcane Fletcher + 1-cost minions (& Halazzi, the Lynx) = card draw mechanic (also have 1x Gadgetzan Auctioneer)
• 1-cost minions + Headhunter's Hatchet = early game answers
• 1-costs minons (& Halazzi) + Toxmonger = mid-game clears (plus Hunter's Mark)
• Zul'jin retakes the board via Animal Companion and Unleash the Beast, while refilling your hand via the beast, rapid fire, and Secret Plan

Against aggro you're trying to curve out a 1-drop beast (Shimmerfly, Springpaw, Boar) into Headhunter's Hatchet into Animal Companion.  Otherwise you'll have to burn some spells such as Rapid Fire or Secret Plan (to find, e.g., Explosive Trap).

Against control you can afford to be a little greedy and search for Arcane Fletcher + a 1-drop minion (ideally Shimmerfly or Springpaw).  However, I'll also keep an eye out for good early-game plays such as Animal Companion, as well as a removal option like Hunter's Mark if I'm worried about giants or big mechs early.

Biggest struggle for the deck is card draw if you curve out poorly - only draw engines are costly/require combos (Fletcher and Auctioneer) and most of your value generators come late in the form of Halazzi, Unleash the Beast, Exotic Mountseller, and Zul'jin (Shimmerfly is your only early generator...though springpaw, rapid fire, and secret plan give you a little too).

Although I like playing with some of the less popular cards, if you're interested in a more competitive build I'd consider swapping 1x Arcane Fletcher for 1x Tracking and 1x Toxmonger for 1x Dire Frenzy. 

Additional considerations include Scavenging Hyena to synergize with the early game minions (would swap the Elven Archer for another Stonetusk Boar) as well as Wing Blast.