I guess it depends on how you define a boss. Ferumbras appeared as early as 2000/2001 and was controlled by the Cipsoft member Durin, in 2001 he appeared in the form of a lvl 145 sorcerer. Aside from being an extremely powerful mage, he also had the ability to summon very tough creatures, such as dragons and warlocks, and during the attacks he would often take on the appearance of a demon.
He would attack Thais on a daily basis, and leave a trail of bodies in his wake. but after a few days our heroes figured out how to defeat him.The final battle was in Plains of Havoc - where Ferumbras ultimately met his demise. Until he appeared as a boss again, in the current form as we know him today, for the first time in August 2006 on Eternia.
I have no idea what he dropped, or if he dropped anything at all. I know there were some very rare spell runes that were said to have dropped from Ferumbras, but I don't know whetever or not they were from this battle.
Here you can see some screenshots of the encounter: http://www.tibia-alumni-society.net/cobalt-tower/Screenshots/ferumbras.html