Short Spine Syndrome is exactly what it sounds like: A shortened spine.
I came across this when I was on snapchat and found an article on The Dodo's snapchat story on a dog named Quasimodo with short spine syndrome.
Link to article here: https://www.thedodo.com/german-shephard-short-spine-syndrome-2314526250.html
I was thinking that if we did have this mutation on Lioden then it would appear at birth and wouldn't be deadly

UPDATE 6/15/17
_________________

Thank you for all of your suggestions on this possible mutation on Lioden!

I am planning on drawing this mutation still (I never got around to it sorry!!!)
Here are some updated ideas though!!

Q: Lethal or nah?
A: No, due to the help of the pride, the mutie would survive such as Squishwolf seen here https://retrieverman.net/2014/04/18/deformed-wolf/ who survived with the help of his pack until a hunter killed him

Q: How rare?
A: Extremely rare, as rare as breeding a lethal.

Q: How is the mutation obtained?:
A: Generations of inbreeding, fatherXdaughter then one of those cubs breeds with their father/grandpa and so on. Again and again and again

Q: Would there be different spine lengths like how there are different mane mutations for females?
A: I would say only two variations, extremely short (like Squishwolf) and short, but maybe only one variation depending on how our Founding Felines think.

Q: Do breeding items help?
A: No they wouldn't help, maybe there could be one made to increase the chance but I don't see it happening.

Any other questions? Please let me now so I can answer them!!