I was writting an answer to a similar question which was set as a duplicate of this one. Since I believe my answer will contribute to this topic, I will post it here.
This is one of those tricky questions which have different answers depending on how you look at it.
If you look at the current bosses in Tibia, the oldest of them in undoubtedly the Black Knight (BK). The sprites used by BK were first seen in version 5.0 (June 18, 1999), the same update that elarged the map which used to contain only Thais to the entire free account continent. However, it is hard to determine whether black knight was already reachable by players at the time, or if this happened a few months/updates later.
However, technically speaking, there were no "boss monsters" back then. So, even though BK was a unique creature only found in a single place on the map, with a slow respawn rate and with unique loot (which later would be dropped by other creatures), it was not an actual boss monster and would not be set as one for many years.
Over a year later, a CipSoft member played a character named Ferumbras - which had its own lore - and basically started to PK people. It had high skills and level for that time, not to mention special spells from a Cip character. Afterwards the devs implemented an actual boss with the same name related to same lore, however this doesn't change the fact that the first Ferumbras was a character, not a monster. Therefore, it should not be considered as the first boss.
Ok, so when the actual first bosses were implemented? That's still not easy to answer. There are traces of the original Pharaohs (which, by the way, are not actual pharaohs in the Tibian lore) on what we can call the Summer Update 2004 (7.3). These bosses known as "pharaohs" are similar to the Black Knight, they have a unique spawn place (which looks like a boss room), unique (rare) loot and they even have exclusive outfits. These were also the first creatures to have their name capitalized and to not have an article (a/an) before it. However, the pharaohs were not reachable because their related quest (The Ancient Tombs Quest) was still under development and would only be released to players 4 months later with Ankrahmun's second update, Christmas Update 2004 (7.4).
A curiosity about Update 7.4 is that it brought the Raid System, and one of the original raids was responsible for triggering the spawn of a creature that couldn't be found anywhere else, had a unique outfit and unique rare loot: the Yeti. However, yeti was never technically set as a boss monster, this decision was possibly taken because they can spawn in groups of up to 3 yetis, not making it "unique". Also, here we can find the first traces of Orshabaal which was not seen till the next major update which apparently implemented its raid together with the other 9 bosses' raids.
TibiaQA.com is a fansite. Please note that the only official website is Tibia.com. The game Tibia and the website Tibia.com are copyrighted by CipSoft GmbH.
