Kitzur Shulchan Aruch, Siman 10, Sief 1
.(וְיֵשׁ לִמְשֹׁחַ הָרְצועוֹת תָּדִיר בְּשֻׁמָּן שֶׁיִּהְיוּ שְׁחוֹרִים (וְלֹא יִמְשְׁחֵם בְּשֻׁמָּן דָּג הַנִּקְרָא פִישְׁטְרָאן כִּי הוּא מִדָּג טָמֵא
It is advisable to frequently oil the straps with fat so that they will remain black. They should not be oiled with the oil of the fishtran fish since this oil comes from a non-kosher fish.
(Translation via ArtScroll)
What is the fishtran fish? I understand this might be closed as it concerns the translation of a word, but I just want to know what this fish is. Obviously the lesson we learn from the above is that the oil of any non-kosher animal may not be used to oil the straps of the tefillin.