The Harvestmath horse looks camouflaged like autumn trees and is bartered only during the Festival events and after completing 3 simple tasks.
1. First, you must be level 35 and have already purchased a horse. You will need to see the Stable Master to trade in your current horse (from where you bought it) to get a reciept.
2. At the Festival Grounds in the northern part of Bree-land (south of the stables) there is a horse race. You will earn a token for winning a race and the restart is every 10 minutes in case you lose. During this short race around the Festival Grounds you will meet obstacles to hurdle or water that slows you down or you can go around them.
3. Earn 6 tokens from any of the Games-masters located at:
Thorin's Hall in Erud Luin - Dyri (bar downstairs) wants you to gather a geode by clicking on the geode pile.
Duillond in Erud Luin - _____ (24.6s, 93.1w) wants you to grab a scroll by clicking on the scroll box.
Party Tree in the Shire - Sawlo Puddifoot (30.6s, 71.6w) wants you to bob for an apple by clicking on the apple tub.
Festival Grounds in Bree-land - Bess Forester (24.6s, 51.4w) wants you to dig up a pumpkin by clicking on the pumpkin patch.
Remember you can only visit each of the Gamesmasters once per hour so you will have to wait or travel to another location. Each game has different quality items to barter for 1 to 3 tokens. The different item qualities are:
Cracked Scroll (1 token), Well-written Scroll (2 tokens), Faultless Scroll (3 tokens), Cracked Geode (1 token), Pristine Geode (2 tokens), Faultless Geode (3 tokens), Small Apple[/b] (1 token), Medium Apples (2 tokens), Large Apples (3 tokens), Small Pumpkin (1 token), Medium Pumpkin (2 tokens), Large Pumpkin (3 tokens).
Once you have all 3 items (horserace token, 6 fall tokens and the reciept talk to the Fall Festival Horse Trader "Will Peartree" (at 24.6s, 51.6w) and click on the type of horse you need.