Trojan babies, of course. How are they born? Where do Trojans come from, exactly? I saw this question implicitly brought up in some other topic, so I figured I'd put some thought into it.
The Legion of Ur mission and the Crest of Ur material state that the Legion of Ur conquered the underworld and was sealed there, and that the devilmites intended to bring them back as Trojans, but the underworld itself isn't necessarily a direct cause. If anything, it's just being used as a pokemon storage box for Trojans at the moment. So with that out of the way, the other two ways we've seen them come to be is from Trojan statues or knights like Arkus.
First, let's address the statues. We know some of the dormant statues make Trojan noises when broken, and that Trojans turn into rather similar statues upon being defeated. So as opposed to Trojans coming from statues, this would lead me to believe that the statues are simply Trojan corpses that got used as decoration rather than being broken for loot; the noises suggest they aren't just simple statues cut from stone, and Arkus being able to revive explains the statues ability to be re-animated. Arkus's ability to revive frequently and not turn to stone may simply be due to the fact that he isn't a full-fledged Trojan and could still speak, etc.
So now we've got Arkus. One thing you should notice is that as Arkus revives more and more, the rock on his back grows, seemingly making him more of a Trojan. His transformation seems to be related to the loss of his team and guilt over their deaths, and he's spent all his time since in what could possibly be the same graveyard his allies were lost in originally, as his allies were killed by dust zombies in a graveyard at depth 26, the depth you fight him at. Combined with the fact that there's a huge legion of Trojans in the underworld, being dead and being around dead things seems to be a rather apt universal description of how to turn into a Trojan.
However, correlation is not causation; just because it's always the case that Trojan transformation is facilitated by continually reviving and being around the deceased, regular knights can run through graveyards and revive all day long without becoming one. Furthermore, Trojans aren't even classified as undead. So what's the direct cause?
The one hint towards Arkus's transformation is his psychology, his guilt in particular. You could say Arkus is incredibly irrational; he seemingly vents his guilt over the loss of his team of knights by killing teams of knights. His depression developed to the point of him intentionally doing wrong for the sake of fueling his masochistic self-loathing, and the only reason he doesn't actually kill himself is simply his sheer sense of personal duty. You could say that nonsensical nature of his actions, combined with the fact that they ended in his death, could be deemed self-destructive. Along with that, Arkus and the only other named Trojan, Maulos, are both in areas populated by lost souls whose only ability is to self-destruct. Trojans could be a metaphor for how being self-destructive and hurting yourself makes you hurt others. In other words, OOO is telling us that if we’re guilting ourselves over something we need to stop horsing around.
instead of statues being trojan corpses, how about trojans are animate statues?
the crystal in their back is a kind of power source packed with a consciousness, which is why when destroyed, the trojan looses all life and turns back into rock.
arkus was simply "infected" with a crystal, therefore causing something similar to schizophrenia, due to a trojan mind invading him.