Please do not be confused. Let me explain why you are looking at two different illustrations of the same scene, of the same story, by the same artist! When sales of the Masters of the Universe books were at their peak in 1985 Ladybird decided to release a large one-off Masters of the Universe annual. The annual featured all news stories with illustrations from Robin Davies, the regular Ladybird book artist, amongst others. One of these stories was titled "Dragon from the Lost Valley". In this story an earthquake across Eternia releases a large stegosaurus from the Lost Valley, deep in the heart of the Mystic Mountains. The heroes have to lead the stegosaurus back to its home, as Skeletor attempts to capture the creature.
In 1986 Ladybird decided to release another of their regular and smaller Masters of the Universe books, titled, "He-Man and the Lost Dragon". In this story the Evil Horde destroy a section of the Mystic Mountains releasing a large stegosaurus from the Lost Valley! The heroes have to lead the stegosaurus back to its home as both Hordak and Skeletor attempt to capture the creature!
It is no real surprise that Ladybird decided to reuse the original story, as it was rather unique. What makes this interesting is that Robin Davies was asked to illustrate the book, having already illustrated the original story for the annual! Due to the success of the cartoon Ladybird asked Robin to illustrate the book in a style closer to the Filmation series, so you can see that Skeletor in the second illustration looks a lot closer to the cartoon model that the original illustration!
(click on the image to see it at full-size)