Like the skull and crossbones icon used for monsters Inconsequentia wants you to kill? No icon once they are dead to prevent clutter. It would be very useful for the times when there is 1 or 3 monsters left but they have teleported out of the zoo and into a wall. Edit to Add: Someone pointed out that a double zoo with 400 total monsters could cause issues, which sounds plausible. Perhaps just the last ten Zoo members would get the icon? I don't know if that would be more feasible, but it is really just the last few monsters that need to be located.
I agree with the suggestion of displaying them when 10 or less are present. Anymore and it would create unnecessary clutter on the mini-map.