Chordion, a fun app developed by Olympia Noise Co is currently free on the App Store. Use it to play chords and melodies alongside an arpeggiator or drumbeat. You can choose to sculpt the sounds to your liking in Chordion or use it as a Midi controller to control other apps like Animoog.
Check out a new(!) Momupro video below of me toying around with the default setup.