The Boss FT-2 Dynamic Filter pedal is an envelope follower (sometimes called a touch wah) which alters the sweep of the EQ based on the dynamics of the note. I imagine they are hard to come by these days. I think I’ve had mine for twenty yearsat this point.
In the usual mode setting, it sounds most trebly when the sound is loudest and then sweeps towards the bass as the note decays, yeilding a wah type of sound for each note that is played.
Do you have a favorite envelope follower pedal? Tell me about it in the comments!