i was thinking a good mob for desert biomes could be tremors. they could burrow around about 1 block underground moving the blocks of sand up as they go, since they have gravity this shouldnt be a problem with having tear marks everywhere. they wont leave the sand but they burrow around randomly. if they catch you you'll sink into the ground about half way and the thing will take a bites out of you till you get out. it wont be a very common mob but once in a while you can see some cruising around. if your within say 5 blocks or so of it it will charge rapidly into you and make some kind of screaming sound. to since it would only be able to travel on the x and y axis this could be avoidable.
That could be fun. A mob that only attacks you if you are carrying cake or cookies or any other type of dessert.