Okay so, I know it's dumb, but here me out. In cattle, there is a type of of calf born called a Amorphous globosus, which is essentially a ball of flesh and fur that is unable to live in any way. These very much not alive calf's are theorized to be the result of a twin fetus not properly developing, resulting in a mere blob rather than a twin. What if there was a lethal mutation based on this type of stillborn calf.

How it may work: It could only be a possibility in litters that were going to be of two or more cubs, a failure for one of the fetuses to fully develop. Being lethal, obviously it wouldn't be able to last very long, only being able to exist for one day before dying, or well, getting deleted since it will already be dead, so freezing it would literally be the only option.

Mutation rates: Obviously, it would be extremely rare, like all lethal mutations. However, unlike most others, the chances of it can be raised if a Buffy or Grain Of Paradise is used on the father or the mother of the cub. This only raises the chances tho due to the fact that a litter NEEDS to contain more than one cub for it to actually even be a chance, possibly making it even rarer than other lethals since none of them require a specific litter size.

I know this is most likely not going to even get looked at, but really, I think this would be an insane lethal that you only get one chance to freeze if you even get it, so I'm rather fond of the idea!