This slug like parasite is actually a symbiote. After attaching itself to a warmblooded host, it feeds off that being. However, its tenticles, growing under the skin, act as secondary nerves, increase the victims nerve responses.
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