The Collins Magazine Annual was not solely devoted to pony literature: it included material from the whole field of children’s books. As far as I can tell from the British Library, it was published from 1948–1955.
Finding the annuals
None of the annuals are particularly hard to find, but pricing is on the eccentric side, to say the least. Reasonably priced copies will generally turn up if you are prepared to wait.
Links and sources
Many thanks to Kay Whalley, who supplied nearly all the information for this section.