This is a topic I have personally been interested in for years. I have a horse, my mustang Tahoe, who has suffered from seizures for more than nine (9) years. He takes daily doses of Phenobarbital and Phenytoin (the equivalent to Dilantin) in order to live a "normal" life. The source of his seizures has always been a mystery. He suffered a head injury once but I've never known if that's what caused the seizures or whether the seizures caused the head injury. From time to time, I've believed that a vaccine reaction could be the source of his neurological imbalance. As a result, I rarely vaccinate him now, and then only one at a time, if absolutely necessary.
It is interesting that more studies have not been done regarding seizures in animals, but especially dogs and cats. Growing up, one of our dogs (Frosty) was kicked by a horse and had seizures thereafter. She was able to live a normal lifespan but had to take daily medications to control their occurrence.
You can read more at PetMD here.
Head Injuries and Seizures in Dogs and Cats | petMD