Revelation’s Beast is equivalent to Daniel’s 11th horn (see here).
That horn comes out of the 4th animal in Daniel 7, which is the Roman Empire (see here).
The 11th horn, in my view, is the Roman State Church. It fits the description of the 11th horn (see here). For example, Emperor Justinian liberated the Roman State Church from Arian domination by defeating three Arian kingdoms. Daniel 7 depicts that as three other horns that were uprooted.
In other words, the Roman State Church, which later became the Roman Church, championing Nicene theology and Sunday observance, is the Beast of Revelation.
Considering the Mark of the Beast, it must be something the Beast is particularly known for. I can’t think of anything for which the Roman Church would be better known than the Trinity doctrine:
(1) The Roman State Church was established when Emperor Theodosius, in 380, through the Edict of Thessalonica, made the Trinity doctrine mandatory for all Romans (see here).
(2) The Roman State Church was liberated from Arian domination when Emperor Justinian defeated the Arian kingdoms (see here).
(3) The Trinity doctrine is regarded as the most foundational doctrine of the Church. It is used as a boundary between insiders and outsiders.
I am not sure we should assume that the Mark of the Beast is evil in itself. Perhaps its only function is identification. Nevertheless, I doubt the Trinity doctrine because:
(1) It teaches that the Son is equal to the Father, while the Bible presents Him as subordinate (see here).
(2) It teaches that the Father and Son are a single Being with a single mind and will, while the Bible presents the Son as a distinct divine Being with His own mind and will (see here).