In this particular case, the translator's note from the NET Bible is helpful:
The meaning of the word תְּחָשִׁים (tÿkhashim) is debated. The Arabic tuhas or duhas is a dolphin, and so some think a sea animal is meant – something like a dolphin or porpoise (cf. NASB; ASV “sealskins”; NIV “hides of sea cows”). Porpoises are common in the Red Sea; their skins are used for clothing by the bedouin. The word has also been connected to an Egyptian word for “leather” (ths); see S. R. Driver, Exodus, 265. Some variation of this is followed by NRSV (“fine leather”) and NLT (“fine goatskin leather”). Another suggestion connects this word to an Akkadian one that describes a precious stone that is yellow or ornge[sic] and also leather died with the color of this stone (N. M. Sarna, Exodus [JPSTC], 157-58).
It also includes a handful of other translations:
"fine leather" (NET)
"fine goatskin leather" (NLT)
"dolphin skins" (MSG)
"fine leather" (NRSV)
The New King James Version preserved the King James rendering of "badger skins", which probably has its own history that might be lost to time.
"Brenton" refers to:
The English translation of The Septuagint by Sir Lancelot Charles Lee Brenton, originally published in 1851.
So it seems that even the earliest translators (the LXX) didn't really know what to make of the word.
This is the speculative portion of the answer wherein I attempt to read the minds of the translators. Caveat lector!
At an absolute minimum, we know from context that the word signifies some sort of skin. (The word it modifies is "skin" or "hide".) It could be a type of animal and, since it was used in the tabernacle, it would make sense that the skin be luxurious. The most conservative translation (in terms of not overstating the meaning of the text) would be to either not modify the word "leather" (BBE) or to modify it with a very general indication of quality (NET, NRSV, GN: "fine").
On the other end of the spectrum, its likely the word being used here was somewhat exotic and possible that some early readers wouldn't have any better idea what it means than we do. To convey the sense of wonder and strangeness that may have been carried by the word, just about any exotic animal, say "ocelot skin", would serve. Since there are etymological and cultural clues that point to a specific animal type, other translators take their best shot at rendering the unusual nature of the skins (NASB: "porpoise", NIV: "sea cows", RV: "seal", MSG: "dolphin").
Somewhere in between are translations that preserve the luxurious but downplay the exotic connotations. It seems they are taking the Egyptian etymology and generalize it to goatskin (ESV and NLT). My reading of these translation is that they presume the word was common (if perhaps technical), but dropped out of use over time.
In any case, I hope all the translations include a note explaining the difficulty with this word and perhaps propose one or two other options.
Your suggestion of transliterating the word is both very conservative (it doesn't say anymore than the original), but also pushes the "exotic" sense of the word. What could be more exotic than tachash skin? I like the suggestion. Perhaps translators don't like admitting they don't know everything about these ancient texts?