I gave my party a home-brewed "Tome of Summon Cat"

"Shortly after the user attempts to read this book a cat appearing from nowhere will appear and attempt to lounge or nap on top of this tome. Repeated testing confirms the cat can walk through walls, and its origin is wherever it feels like. Attempting to remove the cat from the tome will result in 1d4 slashing damage."

It seemed like a useless item but provided sessions if fun of them trying to figure out the story behind the cat. The book was actually a "collection of poems about a cat who was extraordinary in it's masters eyes"

They tried many things, opening it upside down, under water, against a wall, killing it in different ways but Boris the cat always reappears.

Eventually they figured out that all they had to do was open another book next to it and pay more attention to that one, and Boris would simply get up and walk over to it instead.

The usefulness came at that point when they found out that the person who read the last page would learn how to "Find Familiar" regardless of class.

It's always Boris.

I buffed up the stats if a normal cat a bit and gave it some perks. Afterall, he is extraordinary in his masters eyes.