"The Paths of the Perambulator" by Alan Dean Foster, A Spellsinger Adventure

Classic Cover Art.

Jonathan Meriweather, trapped in the strange world where animals walk and talk, and where he, as the Spellsinger, possesses unearthly powers, has faced any number of perils since Clothahump the wizard called him from his own world to help in the war against the evil of the Greendowns.

Now Jonathan, with Clothahump and Mudge the Otter, faces the greatest challenge of all, for the mysterious Perambulator is threatening the very fabric of the universe—unless Jonathan and his company can stop him.