Actually, you first start getting discounts on mounts at guild level 15, then it continues up through level 30 at increments of 5.
Discounted mounts at Guild Level 15 and above
Parade halasian draft horse
Parade steppes pony
Halasian draft horse
Steppes pony
Discounted mounts at Guild Level 20 and above
Parade karana planestrider
Parade misty mustang
Karana planestrider
Misty mustang
Discounted mounts at Guild Level 25 and above
Parade rujarkian clydesdale
Parade elddarian charger
Rujarkian clydesdale
Elddarian charger
Discounted mounts at Guild Level 30 and above
Maj'dul enchanted scroll
Spirit steed summoner (for Qeynos alligned citizens)
Undead mount summoner (for Freeport alligned citizens)
Found on the EQ2 Official Forums