Shannon Gilligan

American author and  computer games designer. Perhaps best known for her involvement in the 'Choose Your Adventure' series, a highly popular series of interactive fiction in which the reader can influence the way the books read by choosing various alternatives. (Similar to the more well known  Fighting Fantasy role play books). Ms. Gilligan wrote many of the books, as did her son and her husband was co-founder of he series. One of the books she wrote in this series involved an equine plot. (See link for full list of all the titles in the series)

She also wrote two other mystery/adventure series for children although I do not think any of these feature horses.

Pony Books:

(BANTAM PB [USA] 1992)
SUMMARY: Part of the 'Choose Your Own Adventure' series of books. 'Role play' adventure game book in which you are a show jumping star about to enter a prestigious competition. However when you hear someone plotting to kill a jumper for its insurance money you know you have to have to help.

Collector's Info:
Paperback editions only. Can be found without too much trouble in the USA and to a slightly lesser extent in the UK. Usually cheap.