I had this idea earlier. There are some genetic conditions that do not show until later in life and are lethal. Why not add a few of those to spice up things. They would:

- Only show up at 2 years of age.
- Have health messages like Lethal mutation cubs except maybe every few days [see below v]
- Plenty of time to be cured [usually these conditions do not kill fast]
- Unable to breed, hunt, patrol or rule a pride unless cured [for balance issues]
- Possibly have their own lineart to show their weak state. [Only one mane type on the males, like with adols.]
- Be very, very rare.

An IRL example would be FOP [Fibrodysplasia ossificans progressiva], soft tissues turning into bone when they get damaged, although it is not adult onset [tends to start at 10 years of age for humans] But by adulthood it would have progressed far enough to be noticable.

There's also certain types of genetic tumors [rats are prone to benign mammary ones that can still kill because of size] and things like Huntington's disease which do show up at complete adulthood only.