Its Japanese name is written as T 666 (Ｔ・６６６), but the furigana characters indicate it is to be read as "Terrible Omen" (テリブル・オーメン, Teriburu Omen).
666 is the Number of the Beast from Christianity. The "T" designation may be taken from the Terminator franchise, where the titular machines are cyborgs like the Three Pure Nobles and are given desginations in the style of "T-600", "T-800", and so forth.
Primo has the ability to merge with his Duel Runner. When that occurs, Terrible Omen transforms, allowing Primo to stand on it and insert his sword to make it resemble a Duel Disk. In this form, Primo and the Terrible Omen are called "Placido Ultimate" (プラシド究極体).
Terrible Omen normally is a 3-wheeled runner with two wheels in front, but once transformed, it becomes a two wheeled runner. It also gains piping and different connections that combine with Primo. Primo's legs disappear, creating two box shaped protrusions where his legs should be.