Is this a Hybrid? (Suggestion and Discussion)

For the first one, Is that really meant to be the concept art of Thunder? I've always assume it's just the unused unrelated dragon or Feather Concept art.

I think it's okay to Headcanon them as such. if there's no info or confirmation, anything is a fair game. Heck I HC the one of the unknown dragon being actually the movie version of Gobsucker and the other one being the movie version of Prickleboggle. There're no confirm so it's still a theory/HC but I think it's similar to how you can HC something as something. 

For me persoanlly though,  I don't think they are hybrids or meant to be hybrids. I think some dragons can exist with the similarity between dragons without having to be hybrids. The clearest example being Scuttlclaw both the final product and the concept art. The official description even state that they look like the cross between Deadly Nadder and Zippleback but as far as we aware, they're not hybrids. 

HTTYD has some signature designs; the limb shape, the bumbs, the underbites, the spines shapes.  fins head. Those would tend to pass around, reused or carry over to other dragons to give the desgin the franchise unity, and one might associated certain shape strongly to certain dragon, like club tail and bumbs for Gronckle which do exist in other dragon like Windgnasher, Red Death or Snafflefang. There's the whole movie 2 dragon gang with the modular system. Design would tend to overlapt. If Goregutter have the moose-like Antler, does that mean future dragon would not allow to have it without it being the hybrids? Are all two-headed dragon Zippleback Hybrids? Are all four wing dragons Stormcutter? Zippleback kinda open the gate way for Seashocker in the second movie perhaps so I think Stormcutter would also basically give a green lit that  four wings dragon can exist too (as we see singetail coming out later)

Songwing (concept art) was made during the era of Book of dragons, right? Way before the Hybrids are being canonize. In the Era of Book of, design also overlap if you think about it; Thunderdrum is the flatted skrill. Whispering Death is the carniveorus plant version of Thunderdrum.  Timberjack is the different font Monsterous Nightmare. So I do think Songwing is its own dragon with the design element that just overlaps  others, that just happens. Maybe all of them original from the same design then being seperate and work on further: Songwing is the big winged Skrill. Timberjack is the Monsterous Nightmare adjuscnet Songwing and so on.

As for Eruptodon, I think somebody has point out before, but that might just be the case of revere type statue having multiple limps and crown.

With all that being said. I don't doubt that if we hybridized Eruptodon and Stormcutter or Skrill and Timberjack. The result would be more or less similar to these dragons. In the topic of "I really like the look of these unuse dragon, perhaps we could hybridize something similar to work as the subtitute" is interesting. Kinda like Cloudjumper original design! I really like the look of it and is the reason why I fell in love with the boltstamper because it reminds me so much of them!

Maybe we could Hybridize Timberjack and Skrill to create the Songwing-like Hybrids if we're not to get the actual Songwing (Concept art) but the hybrids and Songwing will remain a seperate dragon that just look similar.

Although for the Nine realms dragon concept art specifically, I don't think just Stormcuter and Light fury would cut it. It has Speed stinger quality in them too and something not existing before.(They're probably original Beta fish inspired dragon that isn't meant to be Hybrids since there's no dragons with this type of beta-y wings and tails )

Do I want to see some more hybrids that look like concept art dragon? Yes! *throw in another Cloudjumper concept art* Could work as the Razorwhip Snow wraith Hybrids I think

I'm a strong believer that the statue in the Labyrinth is DEFINATELY a hybrid. It's just to obvious to miss. However, I haven't seen any evidence for WHY a hybrid like that would show up as a deity to the creators of said labyrinth