I found the command to make NPC's non-essential but

[anonymouselbow]

I wanted to kill Barbas, but the dog takes no damage even after I used the command to mark it unessential.

[Arthmoor]

Barbas is likely in several aliases which have him marked as essential. My understanding of things is that an NPC in an alias cannot have that status overridden as long as the quest the alias is part of is active.

[anonymouselbow]

Thanks, I was not aware of that term alias before so now I have something to go on.

When I typed help Serana 0 in the console the result included that term alias multiple times.  But, Serana was killed before I got her home, so that may be the difference that Barbas had an alias in more than one active quests, whereas Serana was only active in Bloodline at that time.

[HazakTheMad]

I thought barbus used a different thingy to make him invincible than an ordinary essential thingy.