My guess is that OP saw a bobcat. They are usually grey in color and are about 18" tall. They tend to be more bouncy than slinky, but if it was stalking something it might slink. If you saw the tail and it was very short, it was almost certainly a bobcat. Bobcats are quite common in Florida, but are not often seen because they are much more active at night. Not so much SSR, but certainly the THV area and the wooded areas north of SSR are ideal bobcat habitat.
Not likely it was a Florida Panther. Adult Panthers are very long (6-7 feet) and low, with a very long tail. Adult females usually weigh 100-120 pounds, and males are somewhat larger (150-160). That is quite a bit bigger than OP described. They are usually a tawny brown in color.
I'd be pretty surprised if there were not a few Panthers in the WDW area. It's an ideal habitat with a lot of deer, which are Panthers' preferred prey. Also, we tracked a Big Cypress female north of I-4 in the WDW area several years ago. Eventually, her radio collar battery died, but there have been no reports of mortality matching her age and sex anywhere in Central Florida so we assume she is still somewhere in that general area.
I'm pretty sure it was not a chupacabra, although SSR would be ideal chupacabra habitat. Chupacabra simply are not seen. The only evidence you usually find is an eviscerated dog, cat, goat, etc. Especially goats -- they LOVE goats!