I found this cover of what I’m pretty sure was a textbook back in the 70s. I can’t prove it, but it seems to be from 1978 and is a reader. See OCLC: http://www.worldcat.org/title/free-rein/oclc/16956747
Was this a codeine dream by the illustrator? We’ve got a winged horse (I don’t think it’s Pegasus, since it has asses ears) standing up to its legs in hay, with Easter eggs about the size of breadboxes , inside a pipped egg that must have been laid by a UFO it’s so big.
Out of curiosity, does any of the content of the book explains what a “mare’s nest” is, since I think that is what the cover is supposed to be showing? I never heard of a mare’s nest being inside a huge chicken egg.