Korg's SQ-1 is a step sequencer that's been specifically designed for the Hz/V system of the MS-20, but can also work with the more familiar V/Oct. Two banks of 8 steps are at your disposal.
The Electro Harmonix 8 Step Program analogue expression/CV sequencer is able to rhythmically control the parameters of all devices with a CV/expression pedal input, such as the EHX HOG 2.
The Electro Harmonix Clockworks rhythm generator/synthesizer is a reissue of the EHX classic from the 1970s. The EHX Clockworks pedal allows you to create the most bizarre soundscapes.