Mary Shillabeer worked mostly as an illustrator of children’s early readers, of which she illustrated a wide range. She illustrated relatively few books for older readers, but amongst them are three pony books. Her forth pony book (Dora Broome’s Circus Pony) is an early reader. None of these books entered the mainstream of pony publishing, all having only one printing.
I’m not sure that drawing horses was Margaret Shillabeer’s forté, which perhaps explains why she illustrated so few pony books. Her drawings for Primrose Cumming’s The Deep Sea Horse, a fantasy about the tail-less horse Claude, set under the sea, has some wonderfully lively fish, but Claud is alas rather stiff and awkward. This was some improvement though from Pony Trek, where the ponies are just plain bad.
Finding the books: many of the books illustrated by Mary Shillabeer were children’s early readers, and so have not survived their school careers and are hard to find. The most expensive book I found when researching this piece was The Real Sky-Blue, by Bela Balazs. Of her pony books, The Deep SeaHorse is hard to find, and tends to be expensive. One More Pony and Pony Trek are reasonably easy to find. Circus Pony is easy to find.