I've seen a lot of folks mention how we need both some adol lethals, and some blind/deaf lions. So, taking all that in mind, I raise you...this unnamed mutation that I've been calling Underdeveloped.

The mutation is something along the lines of the top of the head failing to properly develop. The lion was born with enough brain to somewhat survive with care, and thus will last under a broodmother, but is developmentally stunted, has a skull appearing to almost be squished/halved, and lacks the eyes and ears. I don't know if this is a real genetic occurrence, tbh, but it seems logical enough to me. If it seems out of place that this would only affect the upper half of the skull, then maybe the legs are malformed, the spine is misaligned, and the tail may be stubbier as well, to continue with the idea that the lion did not properly form in-utero, but did form enough that they can somewhat survive for a little bit.

EDIT: This would be comparable to Treacher Collins / mandibulofacial dysostosis with the addition of undeveloped eyes.

(if this seems real familiar to you, its because I suggested this on somebody else's mutation, but felt it was unique enough to have its own forum post)
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Pose Ideas (Because What's A Lethal Without A Unique Pose?)
(My phone is being a butt and I'm too lazy to grab my notebook, so here's the best written descriptions I've got)

Cub Newborn - 'loaf' pose? Mouth open, 'confused' expression.

Cub Young - Standing, rear is lower than upper with legs more splayed, front paw up, 3/4 view.

Cub - Sitting stance, splayed legs, 'attentive' face.

Adol Female - Upright/standing stagger-stance, head tilted slightly to the side and up, mouth open.

Adol Male - Upright/standing stagger-stance, shoulders are hunched, head is drooping.

(Artists of Lioden - feel free to draw your own versions. They'll def be added here!

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Images! Art! Pose References! Photographic References!
...it'll exist eventually. I'm a fulltime college student, my time is limited, sorry!
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Suggestions From The Jury (Aka Lioden's Userbase)

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Closing Remarks
Support? Don't support? Have additional ideas? Please, tell me! But I do request that you refrain from not supporting (or at least refrain from commenting) for the following reasons:
-It would add more work onto the Lioden staff (Every single suggestion would add more work onto the staff team. Ever. Single. One. Plus, its not like this mut is being demanded rn. I will willingly wait a decade to see this in-game.)
-You hate me and my other ideas, so you refuse to entertain this one. (No.)
-You hate mutations, period. (Why are you here?)