I have not been able to find any authoritative source that identifies these two fish. It seems that - like many plants and animals mentioned in the Talmud - we have lost the Mesorah that identifies them. One thing we can say is that the Sea Donkey must be a type of fish that has scales (necessary for every Kosher fish) but they are hard to identify either because they are small or they don't grow right away or they fall off when it is taken out of the water (the Beraisa tells us on the same Daf that such a fish is Kosher), otherwise we wouldn't need the Gemara to tell us that it is Kosher. Similarly, the Sea Ox must be a fish that has something that resembles scales and without the Gemara we might have thought it was Kosher.

Kol Tuv,

Yonasan Sigler