I'm on my 2nd playthrough of Borderlands and I have only 30 guardian kills towards the 'What is that Thing?' guardian kill challenge.
I've noticed that at the end portion of the game, you kill a bunch of those Sera Guardians, but they don't bump your guardian kill count for the challenge.
According to the wiki:
A Sera Guardian is an aerial form of Guardian, pink and greyish-green in colour, that levitates high in the air during combat.
I understand that killing Boz Scaggs doesn't count against your Skag kill count, but these things are Guardians.
So, why don't they count?