noun
runt
RUNT/rʌnt/
An unusually small or weak individual, especially the smallest in a litter.
The runt nestles beside three noticeably larger puppies.

Etymology
The origin is obscure. Earlier animal uses referred to small cattle and then undersized animals generally, before the word specialized to the smallest member of a litter.
In context
I do have experience with what amounts to having a runt in the litter
More examples
- They fed the runt separately.
- Several litters included runts.
- The runt soon gained strength.
Forms
runt · runts
Similar words
- weakling
- Emphasizes lack of strength, not necessarily small size.
- underling
- Means a subordinate person, not a small animal.