the runt of a litter

Runt is a relative matter; the smallest of a litter is the runt. Sometimes there isn't one that is noticeably smaller.

It depends on a matter of degree. A non-viable runt piglet can certainly be seen as such in isolation. They are not just tad smaller than the others, but tiny, almost not fully formed and feeble.

A runt isn't just the smallest of the group. If that were the case, every litter would have a runt. To qualify as a runt, the individual has to be abnormally small and weak, which would be immediately noticeable to someone familiar with the young of that particular animal.

It's like the difference between a full-term 8 pound baby and a preemie. You can tell at a glance that the preemie is smaller and weaker than is ideal, whether there's another baby around for comparison or not.

I remember learning back in mammalogy that littering animals will often allocate different amounts of resources to their offspring in utero (and after they're born too), especially when a litter is large. This is because when conditions are uncertain, it's unlikely that all of the young will survive no matter what. So having one or two that are a lot smaller makes it more likely for the others to survive and also makes it less likely that the mother will "waste" more of her energy than necessary on a non-surviving pup or kit or whatever. But if resources are abundant, it's possible for the runt to make up the size difference as he/she grows. Sometimes an animal that was a runt ends up being a normal-sized, or even a large-sized, adult, but under natural conditions, it's more common for the runt(s) to die.

My cat was super tiny. People thought she was only a kitten since she was so small. I guess it depends. My cousin had a pitbull that was the runt and while it was visibly smaller you had to know that she wasn't still growing.

It's very common with dogs to have one that is smaller than the others, though I wouldn't always call them a runt. A dog uterus has two long horn shaped structures that the puppies grow side by side in. The puppies are the end of the horns are often small compared to their siblings if not enough nutrients get down to them.

My last litter had several with a 17 oz. birth weight and one with a 7 oz birth weight. The difference was massive, but by the time they were 9 weeks old she was just as big as her siblings.

A friend with the same breed had another litter with complications. I think her's was a true runt in that case, he was only 4 oz. The mother's uterus actually torsioned during birth and one puppy at the end of a horn was cut off from blood and oxygen until the emergency vet got him out. He was resuscitated, but was so fragile nobody thought he would live. She was very diligent in hand raising him and he was a fighter. He's 2 years old now, still a little smaller than his siblings, but surprisingly didn't suffer any ill effects from the lack of oxygen. He's everybody's favorite.