[To revert to] the main text: 'R. Eleazar propounded: Can the skin23 of an unclean animal be defiled with the defilement of tents?'24 What is his problem?25 — Said R. Adda b. Ahabah: His question relates to the tahash which was in the days of Moses,26 — was it unclean or clean? R. Joseph observed, What question is this to him? We learnt it! For the sacred work none but the skin of a clean animal was declared fit.I discuss the identity of the tachash here. In this parshablog post, I discuss how Onkelos understood tachash, as sasgona. This might be an animal or a color. In a follow-up post, I discuss how Rav Yosef understood tachash as sasgona.
Perhaps the most important quote from those posts:
Turning back to Onkelos and sasgona, I would would note that sas is worm in Aramaic and thatgevan is color, such that it may simply refer to a color of the sas worm. I could also point to the word argaman, which is Aramaic is ar-gevan, perhaps of similar construction. So, Onkelos might not be intending to highlight a particular animal with this, or endorsing this Rabbi Nechemiah beast of multiple colors theory, despite how Rav Yosef interprets sasgevana in the gemara.
This is what Jastrow (page 1009) has to say about the word: