The Centaurian Defender is the only monster in the Taurarus boss tower, aside from the Centurian King. They defend the Centurian King, as their name suggests, and will kill new players unprepared for their large numbers. Within the tower, they spawn in the large corridors and inside the boss room. The speed of a Centaurian Defender makes it difficult to outrun it, equaling even a player wielding Frenzys while equipping Rogue's Plate.
The Centaurian Defender is what makes clearing Taurarus so difficult, as few can stand before the large hoard of Centaurian Defenders in the boss room alongside the Centurian King.
Another problem with the Centurian Defender is that there could be a chance that you could trip and the Centurian Defender sits on you. This also goes with the Centurian King.
A guide exists on the wiki to show how to fight the Centurian King alone: