Is it illegal to let animals go in the wild?

depends on the animal, it's illegal to release an animal that is not indigenous to the area.

Depends on the animal, and country. I live in Canada, you can do it to a dog or cat, but why would you want to? That's animal abuse

I don't know if it's illegal but it sounds wrong

In some cases its not XD not very many though. Think about it, in the UK people can legally take tadpoles out of a pond and raise them into frogs but then have to set them out into the wild. If you have a dog you can't do that though (Unless it ran away)

It's usually illegal. It's also cruel. Unless this is a rescued wild animal that is bounding with energy, seeking to be released back into its native habitat, having been raised by a wildlife rehab expert, like nooooo, don't release domesticated animals into the wild. Most often, they can't fend for themselves. Those that can fend for themselves, may, indeed become a huge nuisance. I kind of think the same could be said for humans, lol. We're pretty much an invasive species.

Read more here:
Exotic Pets Turn Invasive, Threatening Florida
http://www.livescience.com/16204-florida-invasive-reptiles-amphibians.html

Burmese Python - The Snake That's Eating Florida
https://www.nytimes.com/2015/04/06/us/the-burmese-python-snake-thats-eating-florida.html?_r=0

What Happens When People Release Exotic Animals Into the Wild
http://www.onegreenplanet.org/animalsandnature/what-happens-when-people-release-exotic-animals-into-the-wild/