
a young dog = puppy

Image: Sigismund von Dobschütz
a young cat = kitten

Image: Krzysztof P. Jasiutowicz (Kpjas)
a young bear, lion, tiger, or wolf = cub
a young bird = chick

Image: fir0002 | flagstaffotos.com.au
a young duck = duckling

Image: Ks.mini
Learn more: English Vocabulary Words for Birds
a young pig = piglet
a young horse = foal

Image: Kumana @ Wild Equines
a young deer = fawn

Image: Guillom
a young sheep = lamb

Image: Mick Lobb from geograph.org.uk
a young goat = kid
a young frog = tadpole

Images: Kabir Bakie / Dawson
a young butterfly = caterpillar

The butterfly is on top of the leaf, and the caterpillar is under it. Image: David DeHetre