Kulu-Ya-Ku:

Less defined body parts, less plumage, and color like how baby chickens are. Fluffier than a grown one, as they molt the scales begin to show more than the feathers.

Pukei-Pukei:

MUCH fluffier than its adult version. Less colorful, the colors that appear on its tail correspond to they different types of mushrooms it consumes. It also develops more colorful plumage as it grows.

Arzuros:

As a cub it has more muted colors to try and be camouflaged to help keep it safe as it grows. Scales aren't fully grown yet.

Chatacabra:

I like to believe it kind of has more of a frog life cycle, so it has a tail, and as it grows it loses the tail and adopts stronger arms and legs.

Nargacuga:

Not really a lot of thought on this one. I just think scruffy kittens are cute and a nice idea for this monster. Also the way kittens tails stick up when they are young feels like a nargacuga thing :p the blade arm also develops as it grows

Paolumu:

Their air pouch is the first thing they learn how to use, so young paolumus stay in their air form so a long time until their wings become more developed. They also tend to be pinker due to a thinner coat of fur.

Diablos:

Having many horns and spikes, young Diablos's are known for scratching, teething, and digging. They are also tend to be more pale, and as they molt their scales deepen in color.

Tobi-Kadachi:

More fur than its adult form. Young Tobi-Kadachi's struggle to control the static in their fur, and are frequently in it's "charged up state". As it gets older it's spikes develop to help regulate the electricity.

Barroth: