The free Mono synthesizer AnaMini, from Lars Rosenquist, now has Virtual MIDI (including Clock Sync In), as well as a 16-step sequencer. The input is a little wonky; it is difficult to get a precise value from the knobs and the step-sequencer input is laborious. That said though, the sounds are great!
Although there is no conventional filter envelope, you can get a nice acidic decay with the VCF LFO rate set to 1.00 (if you can set it to 1.00, I gave up and left it at 0.98) and the amount set into the negatives. Pushing the rate up to between 2-3 makes for a delayish sort of effect. There is a demo video from the beta, but it is kind of lame so I made a little audio demo.
If you do pick this up, the first thing you're going to want to do is swipe to the left (to go to the right panel) and hit the MIDI connector button in the bottom right. There you'll find the Gain and you'll want to bring that way down! It was deafening with my headphones down to 2 notches on the iPad's volume control. As you see in this screenshot though, it has a nice Virtual MIDI implementation. This seems to be a free version of a larger app that is still in development, so it is fairly awesome to get a free synth with MIDI and a sequencer as the demo!