Here are what i found in the internet about the third legendary Nidhogg (if it were real
Deep within the earth, in the misty nætherworld of Nifelheim, lies the great dragon Nidhogg. It lies, coiled around the well of Hvergelmir, from which all the waters in the world flow, and chews on the roots of the World Tree, so that it constantly dies and is reborn. The dragons attention wavers only when it stops to gorge itself upon the corporeal remains of the dead (for its name means corpse swallower, or the lower one), and to hear the pronouncements of Wyrd when the volva speaks.
=>> the third could represent GROWTH, Birth or Immortal
Nidhogg is the chthonic counterpart of the aquatic serpent Iormungand, but unlike her, and all preceding Rökkr beings, the origins of Nidhogg are totally obscure. The dragon, like the World Tree on which it chews, appears to have arisen out of Wyrd itself, having no creation, no creator. Suggesting at least that they both began their existence so remotely in the subconscious that they have gone beyond the need for any explanation. Certainly, Nidhogg has gone beyond the need for definition. It is beyond gender, it is neither feminine, nor masculine, but not like the undivided lunar nature of Hela, or the androgynous solar nature of Loki. Instead, its nature is so incomprehensible, so chaotic, that it is simply beyond any such definition.
==>> the third could have no creator and OF COURSE genderless
Consequently the element associated with Nidhogg is Chaos, and a total summation of its nature, while the corresponding planetary sphere is the chaotic Saturn. Nidhogg is, further, represented by the traditional constellation of Draconis, as it winds its way around the North Pole, just as Nidhogg is coiled around the axis of the world tree.
==>> this could be its type
Nidhogg is in continual, and torrid, communication with an eagle that sits amongst the highest branches of Heldrasil, the World Tree. Ratatosk (branch-borer) the squirrel runs up and down, back and forth, along the tree, exchanging insults and gossip between the dragon and the eagle, hoping to stir up trouble between the two. Ratatosk plays the same role as Loki does amongst the gods, causing trouble between powerful forces, but, at the same time, communicating between hel and heaven, and, thus, assisting the flow of the kozmos, and the flow of Wyrd.
==>> there could be a squirrel legendary like Mew (dark/psychic maybe??) and because Yvetal and The third hate each other, so Yvetal type could be stronger or weaker than The third
Although Nidhogg's constant chewing on the roots of the World Tree will eventually shake it to its core at Ragnarok, and although it is a force of absolute chaos, the dragon still performs a pivotal role in Northern, and Rökkr magick. The dragon is an embodiment and expression of the geomantic energies that run throughout the earth, and in particular along ley-lines, and at the sacred megalithic sites that populate western and northern Europe. These earth energies are often regarded as dragon energies, because like the dragon, their power is forceful, almost unknowable, and resides deep within the earth and in stones. One of the most famous of such places is the so-called Saint Michael's line, which runs directly through a number of dragon-related sites in lower England.
==>> dragon/ground or dragon/rock maybe??
Here is the poem ENCOUTERS WITH NIDHOGG
ENCOUNTERS WITH NIDHOGG
I walk into a large garden
There is a river, with a waterfall -
Ear glistens within it
Behind the waterfall - a cave
Walking through the cave I reach a passage,
Spiralling downwards -anticlockwise.
I reach the end of it and enter a wasteland.
*
A.N
*
I wander into familiar surroundings.
Into a cave, a giant troll-like figure stands guard,
Eventually letting me pass.
I walk down the spiral passage.
Nearing the end, the ground starts to rumble.
I find myself riding a giant WHITE serpent.
Exiting the passage, I fall off,
and the serpent slithers down a giant hole
into the ground.
Kalos is based on France, and the France flag has 3 colors : Red, Blue, White. Xerneas is blue, Yvetal is Red, so the third could be WHITE