FlipPad is the iPad/iPhone version of the DigiCel FlipBook. It’s still fairly limited and some of FlipBook’s features aren’t there yet but it’s getting better with every update and it’s priced accordingly. In fact, the basic version is free. The in-app upgrade to FlipPad Pro gets you support for the Apple Pencil, more colors, more layers, more frames and more resolution, all for only 2 bucks.
You can find it in the App Store.