You can get some idea of how close your steed is to advancing in power by
looking at it. The mane and glow will both indicate how close they are to
advancing. As the mount grows in power, the mane and glow will advance to
100%, then go back to 0% repeatedly before you see any change.

 unnoticeable 0-10
 dim          11-25
 ambient      26-49
 bright       51-75
 radiant      76-85
 eye-blinding 86-95
 pulsating    96-100
Mount Races and Auras

The race and aura of the mount will advance at different rates, and depends a
bit on if you?re killing lots of little things or lots of big things. In the
end everyone gets where they?re going though.

Race        Aura
----------------------------------------------------
Donkey      Anorexic (This stage doesn't last long at all)
Pony        Feeble
Camel       Shaky
Zebra       Solid
Horse       Muscular (Definitely get saddlebags by now)
Unicorn     Pulsating 
Wyvern      Fluctuating (Wyvern and higher can fly, even over lava)
Wyrm        Shimmering
Chimera     Flaming 
Nightmare   Radiating 
Abomination Chaos emitting/Wildly Flaming