HOW THEY WORK

Right click an animal with a crate to store them in the crate (This makes the crate very heavy, you'll have to put it on your back)
Place it on the ground like a normal block, and shift-rightclick it to let the animal out. This destroys the crate.
If you want to pick up the crate, simply use an axe to break it. If there is an animal inside, it'll stay in there.
The crate when in your inventory will say what is inside
When placed in the world or right clicked the animal will make a noise to tell you what's inside.
Now you can load your animals up on your ship and sail away! Or it gives you a quick and easy way to get a single animal across harsh terrain on your back (Although I already used the Udary mod for this purpose, never hurts to have options)

FUTURE PLANS

This is just a simple mod I whipped up to fill a need, but I have a few ideas I'll likely implement in the future

Metal Cages - Always needs to be carried on your back, even when empty, but doesn't break when opened
Visual cues - Labels and potentially glimpses of the animal inside the crate, so you don't have to pick it up or hit it to know what's inside.
Multiple Small Animals - Chickens and bats don't weigh much or take up much space, may make it possible to carry multiple in a single crate.

(WARNING: Don't release the animal when the crate is beside other blocks, the animal may suffocate)

This code is released under the MIT OSS License. If you want to use it in a modpack go ahead.

Animal Crate