In the trailer, we see what might be Tsaesci, only it's black and not gold scaled or beautiful. Oh, and 3 arms? Not Tsaesci? Thoughts?

It might help if you post a link to which trailer you are talking about.... there are several trailers now.

http://www.youtube.com/watch?v=kybLYzPb6cU&feature=player_detailpage#t=60s

Looks like the creature is in Coldharbor so I don't think it's a Tsaesci, it could just be a new type of Daedra. But I don't know much of the lore, so I don't know what Tsaesci are suppose to look like, I'm mainly just going off the background and it just looks a lot like Coldharbor.

That could be a corrupt version of Tsaeci. After all, Coldharbour is a corrupt copy of Nirn. Theoretically, you could encounter any creature from Nirn there.

True, because we saw some corrupted Spriggans when they released their Coldharbour video a month ago.

It could be a Lamia.

Those things kept me out of the water.