Well, the horse seemed completely useless for racing; Smiley describes her as "so slow and always had the asthma, or the distemper, or the consumption, or something of that kind".
Another unusual thing about her is that in a race, she always started off way behind, but then right near the end picked up her pace and won by a neck's length (coughing and sneezing the entire time).
The metaphor that people used for her was "fifteen-minute nag", referring to the fact that she seemed so slow that it would take her 15 minutes to finish a race. Besides metaphors, Smiley describes her picking up the pace in a very descriptive and humourous way; read through it in paragraph 5 or so-there's some great imagery in there. I hope that helps!
The first metaphor used to describe the horse is "the fifteen minute nag". This is because she looked so sickly and slow that people would give her a head start in a race. She would let the other horses pass her up and then come racing in fast and win the race at "just about a neck ahead." The imagery used to describe her is also comical. She would "come cavorting and straddling up and scattering her legs around limber,sometimes in the air and sometimes out to one side among the fences, and kicking up m-o-r-e dust and raising m-o-r-e racket. . ." All of this allowed Smiley to win quite a bit of money.