Aliasing mugshot → mug_shot
Link to alias

Reason:

Same thing. Wikipedia article titled Mug_shot

Some posts tagged with mugshot, while others tagged with mug_shot.

Other way around (mug_shot -> mugshot) also ok,
but I do prefer mug_shot as a tag.
(Counting text in the body of the Wikipedia article:
"mugshot" is used twice,
while "mug shot" is used eighteen times)

BTW, some uploads tagged with mugshot were actually not mugshots (mostly portraits). Possibly some confusion between "mug" (slang for face) and "head" (headshot_portrait).

EDIT: The tag alias mug_shot -> mugshot (forum #271983) has been approved by @Rainbow_Dash.