These are the horse breeds available on Howrse as of 6/30/13 listed by popularity with Arabians, Paints and Thoroughbreds being the most popular. 

Arabian Horse                                 Paint Horse                      
Thoroughbred                                  Friesian                            
Quarter Horse                                  Gypsy Vanner                 
Purebred Spanish Horse                  Appaloosa                       
Hanoverian                                      Irish Hunter                    
Marwari                                           Mustang                          
Shetland                                           Chincoteague Pony        
Austrialian Pony                              Newfoundland Pony      
Standardbred                                   Connemara                     
Welsh                                              Canadian Horse              
Hackney                                          Shagya Arabian              
Lusitano                                          Donkey                           
Fjord                                               Knabstrupper                  
Nokota                                            Brumby                           
Trakehner                                       Curly                                
Morgan                                           Quarter Pony                   
Peruvian Paso                                 Barb                                
Russian Don Horse                        Holsteiner