runt

https://doi.org/10.1351/goldbook.11190

Smallest and weakest animal of a litter.

Note: A runt may be disadvantaged in competing with its siblings for resources and its mother’s attention.
Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 802 [Terms] [Paper]