What are some species of freshwater mammals in texas?

Question

I'm in Brownsville, Texas, and there is what seems to be a freshwater lake near the place I'm staying at. We saw two creatures, similar to the size of an otter or maybe a beaver, swimming in the water and on shore but couldn't identify them as it was dark out with little light. They seemed to swim very quickly and were darkish in colour.

Answer 1

It was most likely beavers, muskrats, or nutria.

Beaver Pic: http://animals.nationalgeographic.com/st...

Muskrat Pic: http://eliot.needham.k12.ma.us/technolog...

Nutria Pic: http://www.bio.tu-darmstadt.de/zoology/k...

I believe river otters are very rare in Texas.  If it was slender and more weasel-like, then you saw an otter ( http://www.nps.gov/archive/romo/images/r... ).  Otherwise, I'd stick with one of the aquatic rodents.