Googling araound a bit, I found this:
The International Standard Bible Encyclopedia
JATHAN
ja'-than (Iathan;
Nathan):
For "Jonathas" in the
King James Version, which is the Latin form for the Hebrew "
Jonathan."
Jonathan was brother of
Ananias and "son of that great Sammaias" (Tobit 5:13).
http://arabic.studylight.org/enc/isb/view.cgi?number=T4872
http://www.bible-history.com/isbe/J/JATHAN/
So JATHAN seems to be a contraction of
JONATHAN, maybe influenced by king
JOTHAM who is sometimes spelled JOTHAN. JA and
JO are to some degree interchangeable with Hebrew names. But this last bit is just a vague theory.
I have tried to find the name Jathan in the verse mentioned above - but without success. Neither in the Septuagint (which has two parallel texts) nor in Vulgate. So I cannot really back it up that way.