The song in question is "Pet Carrot" by Palehound.
You can hear the guitar right at the beginning of this video:
I thought it was a bass at first. Anyone know the tuning?
The guitar is not just tuned in a standard tuning which is transposed down. From what I can hear and see, the low three strings are definitely
C G D
I believe that the high strings are just standard tuning, which would make the complete tuning
C G D G B E
which is not one of the standard open tunings but a mix of drop C and standard tuning.
The tuning of the low three stings is obvious from the intro riff:
-------2-------------2----- ----0-----0------3-------3- -0-----------0-------------
Sounds like a baritone to me. They use a slightly longer scale length, and thicker strings, and one of the many tunings is C F Bb Eb G C, so standard chord shapes can be used. Not as low as a standard bass guitar - in fact the bottom 4 strings (in B E A D tuning) are an octave above normal 5 string bass guitar tuning.