The tag alias #74309 null_butt -> monobutt is pending approval.

Reason: They seem to mean the same thing. null_butt isn't used by many posts, but given the prevalence of null_bulge I wouldn't be surprised if more people assume that's the right tag and use it in the future.
So aliasing it would mitigate that potential problem before it occurs. (That said, personally I think null_butt is more fitting as a tag, but monobutt is clearly the more popular of the two)

I can see how one could interpret that tag to mean a butt which doesn't have a visible anus in a situation where an anus should be seen, instead.