I have an area of milkweed on the edge of my property so I can get a fair amount of Monarch butterflies. Along with others.

Many years ago I found a green cocoon hanging under the leaf of a milkweed. I didn't know what it was until I looked it up.
They are quite fascinating to look at. With the gold dots and all.
The lifecycle is even more fascinating as you can see here.

I watched this cycle a couple different times. Parts of it are sped up.
They all were healthy when they came out and I released them as soon as the wings were finished pumping up.

You have to have patience and watch them very closely because once it starts you get one shot at it.
I made a box and put the caterpillars in with some milkweed and they did the rest.