I found him once but I can't find him again.
his tent moves around randomly between some of the tiles on the northwest of the map, in a horizontal line across the very northern edge
What's the the purple part of the mutation bar for?
they're counting the 'animal' mutations, as opposed to the 'ĝheist' ones which are yellow. the vast majority of mutations are ĝheist, though, so it's entirely possible to play through the entire game and not get a single new 'animal' mutation.
how do I get those?
there's brief explanations for how to get various tfs in the 'walkthrough' section on the title page; the ones marked as 'animal' in the code are 'knotted dog dick', 'six nipples', and 'coarse fur across your forearms and calves'.