Run Wild, book 6: Run for Cover
Tom McCaughren1999

The scientist in the land across the river had no idea what he was starting when he began experiminting with his gene machine. Now the land of Sinna is being invaded by horrifying creatures - weeds big enough to eat a rabbit, a pig with two faces, a bird with two heads - and a terrible predator half-fox/half-cat, that is stalking the foxes.
When two fox cubs go missing Hop-along realises that it is up to him to lead Vickey, Black Tip, Skulking Dog and the othr foxes on a quest to save them all.