Some names are popularly used in a different form than the way they're spelled or pronounced in Tanach. A few that I can think of are:
Nochum (נָחוּם) - is written in Tanach (Nah. 1:1) as נַחוּם (with a patach under the nun).
Yerachmiel (יְרַחְמִיאֵל) - is written in Tanach (Jer. 36:26; I Chron. 2:9 passim, and 24:29) as יְרַחְמְאֵל (with a sheva under the mem, and no second yud).
Yeruchom (יְרֻחָם) - is written in Tanach (I Sam. 1:1; Neh. 11:12; I Chron. 6:12,19, 8:27, 9:8,12, 12:8, 27:22; II Chron. 23:1) as יְרֹחָם (with a cholam following the reish).
Where do these variants come from? And in the case of Yerachmiel, where the spelling (not just the pronunciation) is different, does halachah recognize the popular spelling as valid for use in official documents like kesubos and gittin, or is the Biblical form supposed to be used?
(There are others, such as Yeshayah/u and Yirmiyah/u, where both forms are found in Tanach. My question is about ones such as those listed above, where all usages of each name are spelled and pronounced the same way each time.)
Some others mentioned in comments (thanks all):
Daniel (דָּנִיאֵל) - is written in Tanach (in the book of that name, as well as in Ez. 8:2, Neh. 10:7 and I Chron. 3:1) as דָּנִיֵּאל (with the tzeirei under the yud, which also has a dagesh).
Basyah (בַּתְיָה) - is written in Tanach (I Chron. 4:18) as בִּתְיָה (with a chirik under the beis). (That may have been influenced by the Midrash, Vayikra Rabbah 1:3, in which Hashem, so to speak, adopts her as His daughter - בַּת יָ-הּ - in recognition of her having done the same for Moshe.)
Shamshon (שַׁמְשׁוֹן) - is written in Tanach (Judg. 13:25 passim) as שִׁמְשׁוֹן (with a chirik under the first shin). (That may have been influenced by the non-Jewish form, originally from the Septuagint.)
Tuviah (טוּבְיָה) - is written in Tanach (Zech. 6:10,14; Ez. 2:60; Neh. 3:35 passim; II Chron. 17:8) as טוֹבִיָּה (with a cholam after the tes, a chirik under the veis and a dagesh in the yud). That one might actually imply a difference in meaning: Tuviah - "the goodness of G-d"; Toviah - "G-d is good."