In September, 2012, MLPchan introduced a [#Mature] tag after input from the userbase. The tag allows certain mature-themed content to be posted on some boards.
By putting the text [#mature] in the OP of a thread on an applicable board, the thread is hidden from view from anyone with the default settings. Threads hidden through the mature tag do not appear as stubs, and are essentially invisble to all users. To be viewed, a user must change their settings to enable such threads.
The tag is available on the following boards:
/art/ - for pony related artwork of a mature nature
/pic/ - for pony related mature images of any kind, picture dumps, etc (Now defunct on MLPchan). This board was merged with /art/
/fic/ - for discussion or review of adult themed fanfics. This board was removed.
/anon/ - for any other type of mature thread
This tag covers mature themed content appropriate for the specific board, but does not include any underage depictions (ponies included), real actual depictions of gore, content deemed extreme in nature, or most non-pony related content.