Spoilers---

Flicka -- Near the road between Whiterun and Rorikstead (South of Gjunkar's Monument) -- She's close to the Giant Mudcrab.
You'll find her papers on a dead Imperial near the Mudcrabs.


Bella -- Black and White paint Vanilla texture -- You'll find her across the river from Dragon Bridge Stables near the tents outside of Chillwind Depths.
Her papers are in a knapsack beside the tent.

Windhelm/Kynesgrove clue -- He's in front of Hob's Fall Cave -- his papers and some leveled loot are in his dead owner's knapsack inside.

Rorikstead/Dragonbridge/Castle Dour/Palace of the Kings -- clues (all letters) -- (Mustang type) -- He's across the water from the East Empire Warehouse at a small camp -- http://srmap.uesp.net/?locx=-53214&locy=94143&zoom=17 (there's some loot in the knapsacks and if you dive down into the water, you'll find a few unique potions.)

Falkreath clue -- Black Stallion Arab type -- He's outside of Brittleshin Pass (the Lake Ilinalta side) -- you'll find his papers inside the cave (knapsack) and a knapsack of his dead owner outside the cave near the horse.

Dragonbridge journal -- (Friesian type) -- He's at Cradlecrush rock. His owner's in full steel plate armor -- you'll find the papers on his body.

Markarth -- journal clue -- Fjord type -- This horse can be a little hard to spot; there are a few enemy spawns in the area and he goes after all of them (he’s not supposed to, but he’s stubborn that way.). He's on or near the path to the entrance of Deepwood Redoubt. His dead owner is at the entrance by the altar.

Solitude clue (in strongbox outside Solitude stables) -- The mare and foal are at the wreck of the Brinehammer. Their papers on board the ship in a knapsack.

Shorsstone clue -- 4 mares and foals are at this burned down farm near Stendarr's beacon and Forelhost -- (Papers are in strongbox inside the burning farmhouse)

Whiterun stables clue (in strongbox outside Whiterun stables) -- The horses are just outside of Helgen at a bandit camp (papers and journal are in a knapsck there) -- (I have noticed this mare and foal sometimes run East towards Riverwood into the snowy area -- towards the cave you escape Helgen from)