Iron Dragons

Iron dragons are an ancient species of dragon in the lore of Dota 2.

Morphology, physiology, and whatnot
An Iron Dragon's form seems to be proof that dragon morphology has hardly changed throughout the species' aeons of evolution. It has the standard form of a dragon, which includes a total of six limbs (two arms and two legs in a quadrupedal stance, plus a pair of big wings) and a kite-shaped tail. Both of its hands had four digits that are composed of three fingers, and one thumb, plus their feet had only three toes. Most of these features can still be seen in modern dragons, such as Slyrak, Jakiro, and the black dragons.

Their most notable features are their metallic scales that act as natural armor and an apparent lack of eyes due to said scales apparently covering them. Rather than a typical Dragon's small scales, an iron dragon's scales were more alike to large, overlapping plate armor. These plate-scales were mostly dorsal—the plated scales ran along their back (also covering the top of their forearms and the upper side of their wings' forelimbs and elongated digits, as well as the tips on each digit of their wings, but not the membrane between them) until their plated scales terminated at the tail, forming a pointy arrow-shaped scale that covered its similarly shaped tail. The scales also fully covered the dragon's head and mandible, making it blind (assuming that it has eyes under the plates). Its fingers are also covered in these plate-scales. The only ventral plate that it has is the single breastplate on its chest.

It's unknown if iron dragons are fire breathing dragons. If they did breathe fire, then they should have an organ called a gullet where they originated their fiery breath as well as gnash and grind swallowed food with iron, steel or other hard gullet material. It's also unknown if they still shared other common characteristics with modern dragons such as fiery blood in their veins, tough but light bones akin to a bird's bones, volatile and explosive drool, etc.

Speculation
In the lore of Dota 2, a dragon's scales are often used to forge metallic armor such as the ones worn by Davion. Scales being turned into metal may not make sense, but the existence of an ancestral dragon species known as the Iron Dragons says otherwise.

It is possible that the Iron Dragons are the answer on why a dragon's scales can be forged into metal stuff. Long ago, dragons in the past were once covered in scales of pure metal, but over aeons of evolution, the metallic properties of these scales were diminished but not removed. Today's dragons appear to have none of their ancestor's metal in their scales, but the traces of their scales' metallic glory still remained. Hence, iron dragon scales are the reason why stuff forged from dragon scales turn out to be metallic.

Descendants
Slyrak (and the unnamed species of dragons he belongs to) might be a descendant of the Iron Dragons. The 's title "Kindred of the Iron Dragon" should be a clear indicator of this, plus its lore goes further into this detail.

It's stated that in the chosen few who are imbued with dragon's blood, such as Sir Davion with Slyrak's blood, its magic runs unusually thick and its effects are enhanced and distorted. Thus the blood's transformative powers (Slyrak's blood and its powers in particular) are enhanced, allowing the recipient to draw strength beyond a single dragon and from the oldest ancestral dragon strains, such as the iron dragons of yore.

The takeaway is that iron dragons are described as an ancestral strain, meaning that other dragon species evolved from this ancestor. If Slyrak's blood can tap into the ancestral strain of the iron dragons, then should be related to them by blood. Furthermore, Slyrak and his own species should be their descendants if this is the case.

Heroes
As stated above, the transformative powers in Slyrak's blood allows the to transform into an iron dragon.

Against civilization
In a forgotten age when they still thrived, the iron dragons once terrorized entire kingdoms and raged wars across the countryside.