For the longest time I've wondered about Malyck and what I personally nickname his "Other Tree" (as opposed to Mother Tree, har har). Bringing up the possibility of there being more Pale Trees was too crazy for me to just forget about, which I sadly can't say the same for the ANET writers, and after many years of it occasionally coming to mind I've come to what I believe to be the most plausible theory about Malyck and his origins.
Before I get into it I don't know if this is a beaten horse of a topic or theory so bear with me. I'm also not saying that this is absolutely certain or undeniable fact, but rather I would simply like to bring up and discuss the plausibility of what is presented.
My theory about Malyck's origins - where he comes from, his special relations to Dream and Nightmare, and the "Other Tree" - is that he is a cleansed Mordrem in a similar example to Mawdrey II and the Pale Tree.
Before you say "Madness!" hear me out; I strongly believe that the Nightmare is connected to Mordremoth and is an abstract echo of his will. The Nightmare's revelry in excess cruelty and destruction vaguely mirrors the same revelry the Mordrem express in carrying out their master's will to destroy everything that is not of Mordremoth. Also, a really strong implication behind the connection between the Nightmare and Mordremoth's relation is the Shadow of the Dragon, one of Mordremoth's champions that appeared in the Nightmare's attack on the Dream in the tutorial.
So, keeping that in mind I would also like to bring up Mordremoth's commanders and their special "champion" status among the Mordrem. It is possible Malyck was originally meant to be one such commander-champion of Mordremoth, and the Nightmare, being the abstract mirror of Mordremoth, recognized this, and the Nightmare court were thus naturally attracted to him and dubbed him the Nightmare's "harbinger".
Being originally Mordrem, Malyck's "Other Tree" could therefore be the only other variety of Pale Tree we've seen period - A Blighting Tree, resulting in his disconnection from the thoughts of his fellow Sylvari and his special status as the Nightmare's harbinger.
This is where it becomes less of a theory and stretches into the territory of speculation;
The only stump I've hit with my theory was the question of how. Someone could have possibly discovered him, just like Ronan discovered the Pale Tree.
The most plausible would be a survivor of the Silverwastes centaurs that found his pod and raised him to become who he was instead of a Mordrem, much like Mawdrey II.
Malyck being discovered and hunted by the Court long before you meet him coincides with this because the Silverwastes is not unfamiliar territory to the Nightmare Court, whom killed the Silverwastes centaurs in the first place. Malyck likely lost his memories while trying to evade them as he claims his earliest memories was waking up on a riverbank.
TL;DR Malyck is actually a would-be Mordrem commander, the Nightmare is the abstract echo of Mordremoth's will, and Malyck's tree of origin was a Blighting Tree.