No one really knows. As Gordon Wenham says in his commentary on the book of Numbers:1
As the different translations make plain, it is uncertain what the outer cover was made of. Hebrew taḥas (RSV goatskin) was taken by the early versions to be a colour. But early Jewish scholars suggested that taḥas was the name of an animal. tuḥas in Arabic means ‘dolphin’ and this underlies the NEB’s translation ‘porpoise-hide’, and the  NIV’s ‘hide of sea cows’.
Other translations have: ‘fine leather’ (CEB; CEV; GNB; LEB; NCV; NET; NJB; NRSV [a note adds, ‘Meaning of Heb uncertain’); ‘goatskin’ (ESV); ‘fine goatskin leather’ (NLT); ‘manatee skin’ (HCSB [a note adds, ‘Or of dolphin skin, or of fine leather; Hb obscure’]); ‘porpoise skin’ (NASB); ‘durable leather’ (NIV); ‘a covering of yellow-orange skin’ (NAB [an older version of the NAB left the word untranslated]). Other suggestions include ‘wether-skins’ (a wether is a castrated ram), ‘brilliant’, ‘hyacinth-coloured’ or ‘dolphin’ skins.2
As the footnotes in some of the translations make plain, the Hebrew word that is being translated is obscure — no one really knows what it refers to any more — so there are a number of different attempts to translate it.
1. G. J. Wenham, Numbers (Leicester: IVP, 1981), p. 73 n1
2. Morris Jastrow, Jr. and Gerson B. Levi, ‘TAḤASH’ in Jewish Encyclodepia (accessed 05/08/15)