Stables in Norrath :

Qeynos
In South Qeynos Stable, near the Antonica Gate.

Freeport
In West Freeport, near the Mender/Bank.

Kelethin
In Greater Faydark, just outside the hut where the Citizenship NPC is, between Orc Hill and Butcherblock.

Neriak
In the New Foreign Quarter.

Maj'Dul
From the Courts (40,000+ Faction required).

Standard Horses:
  
Steppes Pony
Regular Price: 2 platinum, 45 gold, 76 silver
Guild Discount: 52,500 status, 1 platinum, 38 gold, 42 silver (Guild Level: 15)

Parade Steppes Pony
Regular Price: 2 platinum, 45 gold, 76 silver
Guild Discount: 52,500 status, 1 platinum, 38 gold, 42 silver (Guild Level: 15)

Halasian Draft horse
Regular Price: 2 platinum, 45 gold 76 silver
Guild Discount: 52,500 status, 1 platinum, 38 gold 24 silver (Guild Level: 15)

Parade Halasian Draft horse
Regular Price: 2 platinum, 45 gold, 76 silver
Guild Discount: 52,500 status, 1 platinum, 38 gold, 24 silver (Guild Level: 15)

Karana Planetstrider
Regular Price: 3 platinum, 68 gold, 64 silver
Guild Discount: 80,000 status, 2 platinum, 7 gold, 36 silver (Guild Level: 20)

Parade Karana Planetstrider
Regular Price: 3 platinum, 68 gold, 64 silver
Guild Discount: 80,000 status, 2 platinum, 7 gold, 36 silver (Guild Level: 20) 

Misty Mustang
Regular Price: 3 platinum, 68 gold, 64 silver
Guild Discount: 80,000 status, 2 platinum, 7 gold, 36 silver (Guild Level: 20)

Parade Misty Mustang
Regular Price: 3 platinum, 68 gold, 64 silver
Guild Discount: 80,000 status, 2 platinum, 7 gold, 36 silver (Guild Level: 20)

Elddarian Charger
Regular Price: 9 platinum, 83 gold, 4 silver
Guild Discount: 112,500 status, 5 platinum, 52 gold, 96 silver (Guild Level: 25)

  
Parade Elddarian Charger
Regular Price: 9 platinum, 83 gold, 4 silver
Guild Discount: 112,500 status, 5 platinum, 52 gold, 96 silver (Guild Level: 25)

  
Rujarkian Hillrunner
Regular Price: 9 platinum, 83 gold, 4 silver
Guild Discount: 112,500 status, 5 platinum, 52 gold, 96 silver (Guild Level: 25)

  
Parade Rujarkian Hillrunner
Regular Price: 9 platinum, 83 gold, 4 silver
Guild Discount: 112,500 status, 5 platinum, 52 gold, 96 silver (Guild Level: 25)

Court Faction Horses:
The following horses require ally (40,000+ faction) with one of the courts in Maj'Dul (ie. The Court of Blades, The Court of Truth or The Court of Coin) 
The regular Rujarkian Destrier and the regular Dervish Destrier are available from the city stables without the faction requirement.
Horses with custom saddle colors require appropriate faction level with one of the courts and are available only from the court you have ally faction with.
      
Rujarkian Destrier
Regular Price: 14 platinum
Guild Discount: 337500 status, 10 platinum, 80g (Guild Level: 35)

      
Blue Saddled Rujarkian Destrier
Regular Price: 14 platinum
Guild Discount: 337500 status, 10 platinum, 80 gold (Guild Level: 35)

      
Black Saddled Rujarkian Destrier
Regular Price: 14 platinum
Guild Discount: 337500 status, 10 platinum, 80 gold (Guild Level: 35)

Purple Saddled Rujarkian Destrier
Regular Price: 14 platinum
Guild Discount: 337500 status, 10 platinum, 80 gold (Guild Level: 35)

      
White Saddled Rujarkian Destrier
Regular Price: 14 platinum
Guild Discount: 337500 status, 10 platinum, 80 gold (Guild Level: 35)

Dervish Destrier
Regular Price: 14 platinum
Guild Discount: 337500 status, 10 platinum, 80 gold (Guild Level: 35)

Black Saddled Dervish Destrier
Regular Price: 14 platinum
Guild Discount: 337500 status, 10 platinum, 80 gold (Guild Level: 35)

Blue Saddled Dervish Destrier
Regular Price: 14 platinum
Guild Discount: 337500 status, 10 platinum, 80g (Guild Level: 35)

Purple Saddled Dervish Destrier
Regular Price: 14 platinum
Guild Discount: 337500 status, 10 platinum, 80 gold (Guild Level: 35)

Pink Saddled Dervish Destrier
Regular Price: 14 platinum
Guild Discount: 337500 status, 10 platinum, 80 gold (Guild Level: 35)

 
White Saddled Dervish Destrier
Regular Price: 14 platinum
Guild Discount: 337500 status, 10 platinum, 80 gold (Guild Level: 35)

   
Guild Level 30 Mounts:
These mounts are available when you are in a level 30 guild.

Spirit Steed
Price: 337500 status, 10 platinum, 80 gold

Maj'dul Enchanted Scroll
Price: 337500 status, 10 platinum, 80 gold
Available from the Mage Tower in South Qeynos or
the Academy of Arcane Sciences in North Freeport.

Guild Level 40 Mounts:
These mounts are available when in a level 40 guild.

Black Saddled Nightmare
Price: 400,000 status, 19 platinum, 20 gold, 1 copper
Available in West Freeport

Red Saddled Nightmare
Price: 400,000 status, 19 platinum, 20 gold, 1 copper
Available in West Freeport

Plain Saddled Nightmare
Price: 400,000 status, 19 platinum, 20 gold, 1 copper
Available in West Freeport

Blue Saddled Mistrunner
Price: 400,000 status, 19 platinum, 20 gold, 1 copper
Available in South Qeynos

Plain Saddled Mistrunner
Price: 400,000 status, 19 platinum, 20 gold, 1 copper
Available in South Qeynos

Purple Saddled Mistrunner
Price: 400,000 status, 19 platinum, 20 gold, 1 copper
Available in South Qeynos

Guild Level 60 Mounts:
These mounts are available for purchase when in a level 60 guild.

War-barded Black Cavalry
Price: 448,875 status, 14 platinum, 36 gold, 40 silver

War-barded Cavalry Appaloosa
Price: 448,875 status, 14 platinum, 36 gold, 40 silver

War-barded Cavalry Morgan
Price: 448,875 status, 14 platinum, 36 gold, 40 silver

War-barded Cavalry Palomino
Price: 448,875 status, 14 platinum, 36 gold, 40 silver

War-barded Cavalry Pinto
Price: 448,875 status, 14 platinum, 36 gold, 40 silver

War-barded Cavalry Spiritsteed
Price: 448,875 status, 14 platinum, 36 gold, 40 silver

War-barded White Cavalry
Price: 448,875 status, 14 platinum, 36 gold, 40 silver

  
Dropped/Quested/Tinkered Mounts:
These mounts can be either looted, tinkered or received as quest rewards.

Carpet of El'khazi
Quested, by starting "Threads of the Desert" given by
El'Khazi, Purveyor of Fine Carpets on the beach by the docks in Sinking Sands

Fitzpitzel's Hover Platform
Drop from Epic Mob 'Fitzpitzel' in Deathtoll

Djinn Bracelet of the Sky
Drop from Djinn Master, The Djinn Master's Prism

Overclocked Gnomish Hovering Device
Tinkered Mount
Required Level: 60

Black Striped Warg with Blue Saddle
Quested, by starting "Dire Wolf Whisperer" given by Heinrich in the Loping plains
Once the Quest is completed he will sell them for: 4 platinum, 91 gold, 52 silver

(More color variations are available).

Fun/Fluff Mounts:

Holy Steed
Paladin fun spell, available at level 20.
Price:12 silver (from the Paladin Trainers)

Unholy Steed
Shadowknight fun spell, available at level 20.
Price:12 silver (from the Shadowknight Trainers)

Glorious Steed
Cost in favor with Marr varies by altar type.
Unlocked once you do the quest "Beacon Of Courage"