Ok, so what on earth could the hidden purple creature have in common with Diablo? The answer might be Michael Dashow. He did a lot of work on Diablo II for Blizzard between 1998 to 2005 and seems to have a special love for purple Walruses.
His Diablo gallery: http://www.michaeldashow.com/diablo.html

Thanks to Xabora for the tip-off!


Update: Blizzard closed down his Concept art gallery with Diablo sketches in 2004 with this reason:

Michael Dashow wrote:Concept art is no longer viewable. Blizzard entertainment has a new policy which forbids the showing of any artwork for projects which have not been released. Even though all of the stuff that was formerly on display here was from long-dead projects which will never be relased, it still falls into the same category and is now considerred to be forbidden.

But looks like the game he did concept art for is very much alive and kicking, here is a snapshot of his gallery page in 2004 before the shutdown:
http://web.archive.org/web/20040806175115...ashow.com/concept.html


In case Michael Dashow is unrelated to the purple creature and it's not a Walrus, these might be better guesses Made by Stn Toulouse.