The Curious Cat Spy Club — Linda Joy Singleton

No one at school knows that Kelsey, Becca, and Leo are secretly friends. They have nothing in common--until they rescue three kittens and form a club to help animals through volunteering, spying, and solving mysteries. In the first book of this series, Kelsey helps catch a runaway zorse (horse + zebra) with the help of another girl from school named Becca. The two are walking home when they happen upon a litter of kittens trapped in a dumpster and Leo Polanski is the only person around who can help get them out. The three unlikely friends decide to work together to help solve animal crimes with their secret club!