Most of the forked blades I can find from historic pieces are katars:
But I have yet to see a katar that has a central tang which we can see on the Arsenic. Also, I can't find any examples of a katar without the vertical bars being attached to the ends of the cross guard. We don't see such bars on the Arsenic.
However, there are some forked blades on daggers with a standard tang and cross guard.
Based on the central tang and cross guard on the Arsenic image, I'm inclined to say it's a dagger. It fits the motif of an assassins weapon used at close range for poisoning.
Forked blades on spears are incredibly uncommon for a few reasons. First, a good spearsman was incredibly accurate, able to pinpoint weaknesses in armor from a distance. That distance is the other important reason. A forked blade is more likely to get stuck in the victim, leaving you attached to the victim and hurting your ability to stay at range.
There are some forked spearhead designs, like the Persian Qajar.
But I just can't find anything similar to the Arsenic. I will say, though, that the central tang on Aarsenic does look somewhat conical and could suggest it's attached to a polearm. But a spear doesn't fit the motif of an assassin's poisoned weapon.
So I would say Arsenic is mostly likely a dagger.
I think the depiction of Scorpio's Arsenic is correct.
