I have a hard time deciding which tier they belong to. Same as elder dragons perhaps?

Stronger than Ancient Dragons. Weaker than Revered Dragons.

There are the third strongest.

I was a tad surprised when they paled in comparison to the legendary dragons, but I suppose it makes good sense what with being legendary and all that jazz. I just wish that at higher levels they'd make a rare appearance rather than costing me all my NPCs.

I didn't know that. I always thought they were on par with Elder Dragons.

They are extremely ugly though.

I wouldnt call them ugly, just different.

Serpentine Dragons are found only on http://www.uesp.net/wiki/Dragonborn:Solstheim and can breathe frost or fire. Their appearance is unique compared to other dragons, most notably their blue coloration and oversized bottom jaws which give them a distinctive underbite. They are formidable dragons to fight, possessing similar strength to an Elder or even an Ancient Dragon. There is one named Serpentine Dragon, http://www.uesp.net/wiki/Dragonborn:Sahrotaar, whom http://www.uesp.net/wiki/Dragonborn:Miraak_(NPC) uses as his mount. Serpentine Dragons begin to appear in leveled lists at level 55, if Dragonborn is installed. All of this came from UESP.net

Really? I've only found one, ever. Besides Sahrotaar.

Are you well versed in dragon beauty and cosmetics?

Still weak enough for me to stab my Dragonbone sword through their skulls on Lengendary.
They're definitely no match when compared to a Legendary Dragon, though.