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1.
A girl who is now a SLUT. A tramp leader basicly who once looked respectable and freash but now may look similar to a raw hamburger(or has the slit of one anyway). A girl that perhaps once, tempted your mind into wanting a peice but you will have to overcome and realise only your brains being an r-tard. A once freash decent girl now called a Shaggy Horse can commonly be first noticed some where(based on popularity) from high school years 8-10. You may tend to keep thinking of this girl in your imagination but you must understand the truth, "I'll probably get some feral disease".
Guy J: "Omg, look at that! what a tramp.. although i would consider it."

Guy D: "Dude, clearly shes been over used... two words mate! Shaggy Horse!"
by Mechlord April 27, 2009
 
2.
Another euphemism, very old in English, for the male genitalia.
My wife rode the shaggy horse 'til it puked.
by butthead April 29, 2006