As of 5.1 a new texture format was provided to devs, specifically for devs making webGL games or anything that requires as little space as possible to make it load as fast as possible.https://docs.unity3d.com/Manual/class-T ... rride.html
Docs here: https://github.com/BinomialLLC/crunch
Using UnityEX, it will identify the textures as DXTn "crunched", and when you export the assets, you'll get a ".tex.crn" file. Sometimes you'll get a .tex.crn.dds, but if for some reason it only converts to DDS, you can use something like thishttps://github.com/Unvanquished/crunch
Which you could then convert to DDS, or TGA, or PNG.