Short series for younger children
1) THE PONY THAT DIDN'T GROW
(IWASHBURN [USA] 1963)
ILLUSTRATED BY RICHARD LEWIS
SUMMARY: Beth is given a Shetland called Merry Christmas (Christy for short) for her birthday. She is told he will grow over the winter but as she waits and watches she can't see him shooting up at all!
2) CHRISTY FINDS A RIDER
(IWASHBURN [USA] 1965)
ILLUSTRATED BY SAM SAVITT
SUMMARY: Christy has not grown big enough for Beth to ride, so she drives him instead. She worries that the pony will not be happy unless he is ridden, then a much smaller girl called Tiny comes along and a solution is found to the problem!
Although not common titles, the books can be found without much trouble in the USA. Fairly hard to find elsewhere.