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There's also The Plague Dogs, also from the Watership Down guy but with dogs, (not cats of course, but bringing it up in case you're misremembering the species.)
Hey! Could it be "Tailchaser's Song," by Tad Williams? Quest story when feline (female) goes missing and must be found. Wise guide shows up & labors must be addressed. Massmarket pbk cover was mostly browns, with cats on ground at the base of a tree, looking up at an "evil" cat with red eyes and nasty, pointy teeth.

I remember hearing about a book that is essentially Watership Down but with cats. I'm not referring to the Warrior cats series. I think it got an animated movie adaptation as well. I can't remember the title for the life of me and my repeated searches of "what is that scary cat book" haven't been working. Please help