Messages app for iOS 10 comes with a neat tool that allows you to scribble and send handwritten message. This feature is build right within and there is no need to install additional plugin or app.
The handwriting ability for Messages app is a little hidden though. To activate it, start by tapping into the text entry box. Then rotate iPhone into the horizontal position.
You should now see a full screen blank panel that you can scribble text on it. There is an even a handy “Undo” button on left-top corner. Once you’ve scribbled enough, tap the “Done” button on right-top corner. This will bring you back to the familiar chat screen. Simply hit the up arrow button as usual to send the handwritten message.
If you can not see the handwriting input panel while on horizontal position, tap the cursive loop icon button on right bottom corner of the keyboard.