if you make a custom digger by giving him/her
when you cast Summon Imp it will create that creature instead of an imp. And that creature's head will be displayed in the creatures tab .
Any already existing imps or any of those creatures created by this bug you will not be able to pick up.
Removing EVIL from creature properties fixes this issue. ODD
I did this with Bug and didn't work properly until i removed EVIL and I did it with Tunneler and worked fine. Both now behave exactly like an Imp 👍 They pretty dance reinforce the walls, they claim, bring dead bodies, dig , and set up traps.