1) What exactly are premature yukkuri?
Yukkuri that have not sufficiently developed during stalk/fetus period. Most commonly due to malnutrition of parent in pregnancy period. Sometimes can be seen randomly without a reason, possibly same way mutations happen.
Also if premature yukkuri is lucky enough to reach adulthood and breed, it would produce premature yus as well. In post #76191 you can see approximately 50/50 ratio due to one of the parents being normal and one premature.
2) How to breed them?
I would imagine easiest way is to starve normal parent yukkuri during pregnancy, and get your first premature batch.
3) How is their intelligence?
Normal (even non-badge) yukkuris are more intelligent than most animals, barring maybe high-end monkeys. They are capable of abstract thought process, planning, imagination and so on.
Premature yukkuris act more on level of low-end reptiles, they can't think and only act upon immediate stimulus. On top of that they do not possess many instincts like recognising danger.
4) How CYUTE are they?
No such thing as objective scale of cuteness. Someone finds them cuter than normal yus, someone feel other way around.
Authors that often draw them usually depict them as appealing as more cute to anons in their stories.
But for reader it really depends on artist style.
5) How difficult is it to raise a premie?
Should be more difficult than normal yu, that recognises at least immediate danger and can voice a complaint about their problem like feeling sick.
Probably still doable with good isolation in yukkarium and close monitoring.
6) Do premies have value?
In most stories they don't possess market value as they are harder to take care of as a pet, and less satisfying for abyuser due to lack of intelligent speech and severely cut emotions.
Rare premie yu lovers usually have to find them or breed them on their own.
Yukkuri parents seem to have same opinion as humans and cull them more readily (although still placing them above mutants).
7) Are they fragile?
It's debatable, but from what I saw they are more often portrayed to have same durability as normal yus. Not always the case, though.
Even if they have same durability, they have fewer survival chances due to lack of intelligence and many important instincts.
8) How often do you see them?
Depends on the author they can range from very common to exceedingly rare. Overall amount of stories where they are even involved is not high.
9) Could they pose a threat?
They are unable of aggression, it's too complex for them. Even if they could, they are even less capable of attacking than normal yus due to bad movement coordination.
Now if you tie a person down securely and cover them in something sweet... even in this case most possible danger is suffocation.
10) Would you expect shithead attitude from premies?
They are too stupid to be a nicehead or shithead. They don't have self-awareness for that. They are emptyheads.