There are several music-making apps for Apple iOS devices (iPads, iPhones, iPod Touch), and drum machines/virtual drum and percussion programs are certainly among them. I’ve written about a couple on this site:
MIDI Step Sequencer For iPad, which is about Nord Beat, a traditional type drum machine, and something that is sort of a virtual drum (and guitar) accessory for the iPad, Pix and Stix for iPad Garage Band App. These are actually drum sticks that you hit your iPad with. Crazy stuff, but may make drummers feel a bit more like they are playing drums.
Anyway, here is a new drum machine for the iPad only (not iPhone or iPod Touch) called Synth Drum Pads, from Paul Betowski. You can create all kinds of rhythm and melodic tones, which seem to be more suited toward the EDM (electronic dance music) genre, but certainly will find use in any music-making kit. Take a listen to an audio sample below:
Synth Drum Pads has 4 large pads, each with a set of parameters (tone, attack decay, etc.) above it that you set/tweak. The guide on the site shows you starting with a kick sound that is very similar to that in the popular TR-808. Note: there are no presets, so you’ll have to learn (or already know) a bit about drum programming. If you’ve used drums machines before, you should be OK.
But even if you have no experience, there are excellent instructions with pictures at the site here: http://designbypaul.co.uk/synthdrumpadshelp.html