What is this giant mosquito-like insect?
The better entymologists among us may need to correct me, but I believe you are looking at a robber fly, especially based on the characteristic long, tapering abdomen, as well as the proboscis that reminded you of a mosquito and the large size.
I see a couple people posting images from Texas that look a bit like yours (especially the all-black body) on Google images but none of these mention a genus/species for me to dig further on. Bromley, S. W. (1934). The robber flies of Texas (Diptera, Asilidae). Annals of the Entomological Society of America, 27(1), 74-113. may be useful but is a bit too deep for me to dig into for you.