While it technically isn't an app per se, it's a third-party keyboard that you can install on your iPhone and use it to implement a drawing pad into the iOS keyboard.
Drawboard lets you doodle sketches within other apps that use the iOS keyboard, like iMessage and other chat apps. When enabled, the keyboard turns into a drawing pad where you can create a sketch and instantly send it to someone, all of which can be done without leaving the app you're in.
It's certainly a unique idea and is something different than just another drawing app, but it's not a third-party keyboard meant for everyone. However, it's at least worth it to try it out and see if you like it.