Cuburt is a named gelatinous cubed he's smaller than the normal ones, and occasionally (1/5 times) drops a small tattered dress that is a quest item in North Qeynos.
Go down into the aquaducts...You have to keep killing all the sewer rats,gellatenous cubes,and mangy rats. They are all place holders for him.Eventually he will show up.It takes from 5 minutes to an hour.Hope this helps.
note: There is a 20 second spawning time from 1 NPC to the next.