"iMessage Chat" has shown up in the Google Play Store, and it seems to be an actual, working, not-very-pretty iMessage app for Android. Unfortunately, since Apple didn't make it, and there's currently no way to look inside the APK file and see what's happening, it shouldn't be trusted.
In order to use it, you'd have to give the app your Apple ID, which could also be the key to your devices, your iTunes purchases, and more. That means your data, that means your credit card information, and that means game over. At least potentially.
It may sound great, but great things are often too great to be true. I don't know anything that could be said or done that would make me trust this - or any non-Apple app - and I can't think of anything that should make you give it your Apple ID. Ever.
Downloaders beware. Actually, don't even beware, stay the #$!! away.