Prehistoric (dinosaur aged) Creatures I think would've been appropriate in Skyrim.
Me here with another set of ridiculous ideas. Buttt...
Given how prehistoric themes were used in Skyrim's wildlife, here are a few creatures I think would've been right at home in Skyrim, had Bethesda had the ability to expand and implement them. Mind you I'm not saying Skyrim should be jurassic park, however there are several ancient creatures I think would have been more than fitting.
Had I any knowledge of 3d modelling and creating mods, I would make them all myself for pc. XD
First up, my personal fav, the koolasuchus:
http://image.wangchao.net.cn/baike/1261675193977.jpg
https://www.youtube.com/watch?v=QjeKEOMH0IM
I can easily pictures these guys alongside mudcrabs patrolilng skyrim's rivers, waterways, and morthal's swamps.
Second raptors: (birdlike):
https://www.youtube.com/watch?v=-fyxUxGdrns
https://www.youtube.com/watch?v=011cZgCSZ9c
These guys are adorable and birdlike enough but archaic looking to really give themselves an edge in skyrim's wilderness. I can just imagine walking down forested roads with one of them swooping down with a poison bite. XD
Next these guys:
http://vignette3.wikia.nocookie.net/walkingwith/images/e/ea/Dimetrodon_eating.jpg/revision/latest?cb=20080528222350
with this texture:
http://kids.nationalgeographic.com/content/dam/kids/photos/animals/Amphibians/Q-Z/spotted-salamander-black-orange.jpg
I would just call them sailbacks, but these guys would be bomb in the volcanic geyser region of skyrim in the east in between riften and windhelm. XD
Lastly, not extinct but another salamander:
http://24.media.tumblr.com/4f30c54ae1fbf9791e865e2d5a1c6268/tumblr_mhk0p97Tki1qdzc7so1_500.jpg
Despite being outrageously adorable, I think these guys would fit in perfectly scaled up to 10 feet in length in Skyrim's caves. While physically weak, unless you put ribcage armor or something, these guys could be threatening by emitting a poisonous gas that drains fatigue and health and burns the slin. Ouch.
Another animal, not extinct would be Narwhals. They's be perfect in the seas north of Skyrim:
http://blog.txapulin.net/img/narval.jpg
That's all I care to write down, theres several prehistory mammals that would be cool as well, but I won't bother listing those. XD
I just think these creatures would fit in nicely and add a great diversity to the landscapes of Skyrim.