When we got my dog insured, we got a good rate because we started her plan as a puppy ($50/year). She was covered for injury and illness, but not for dysplasia or a few other genetic defects. It also did not cover her annual shots/checkups, which were mandatory to keep the policy in effect.

In her old age, the annual rate has increased to $160/year, but it has substantially offset the costs over the years for treatment of her chronic ear infections and countless bouts with rats/raccoons/possums.
I'm not sure the reasons, but the hospital did not offer plans for cat owners. Are cats more prone to disease and injury? I dunno, I'm not a cat owner.