Myrtle Ellen Green wrote two pony books I am aware of: the wondrously titled A Pony, Doctor’s Orders, which sounds more as if it should be a Mills and Boon, and The Part-Exchange Pony. Both books were part of the Crown Pony Library.
Finding the books
A Pony, Doctor’s Orders is easier to find than The Part-Exchange Pony. Both titles are usually reasonably priced.