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1.
Meaning awesome or incredible, often when refering to heavy metal bands. Term originates from song by Dethklok.
"That metal band was thunderhorse!"
by samalot June 16, 2008
 
2.
A very large male reproductive organ, a.k.a a thunder stick or a very large "meat stick"
I jumped on his thunder horse! He put his Thunder Horse in me!

His penis is in the Thunder horse category
by Meat Stick July 14, 2009