What if some crazy wizard created an artifact that empowered rodents that touched it with one random supernatural power? Preferly right out of the pages of your favorite four-color. (But only one power!) This item does not convey any sort of intelligence boost.
Somewhere along the line, someone very powerful lost their marbles. Literally. Possessing a marble allows incredible clarity of thought about a particular related subject.
Next time you're contemplating a horse variation, but don't want to get too dramatic, how about a Zorse? An offspring of a zebra stallion and horse mare. In nature they are infertile, but in a fantasy world, not necessarily. They are also known to be extremely cantankerous.
"It soon became apparent that zorses are not the most easiest of the equine family to get along with." -- Trainer Pat Parelli, on working with zorses