This is the second year since Blizzard is adding Crabby on the official WoW forums, on both of the European and US forums.
This can be done within a simple click.
On good PCs, Crabby should cause no problems. If you have a big monitor, he will not be displayed above the page so browsing the forums should be fine.
For jokes, check the WoW sub-reddit page, reddit.com/r/wow or the arenajunkies forums.
Hopefully, in a few days, Blizzard will decide that is time to remove Crabby.
Update. Crabby was removed by Blizzard a few hours ago. We might see him back next year.