Unicode defines several characters for the macron below:
There are many similar marks covered elsewhere:
Various precomposed letters with a macron below are defined in Unicode:
There is no precomposed upper case equivalent of ẖ so it uses a combining macron below instead: H̱.
Used in Tlingit and Haida (among other Pacific Northwest languages) for the voiceless uvular stop [q]. Close to Korean ㄲ kk; closest English "shocking"
Used optionally in the K-dialect of Māori in the South Island of New Zealand, where an original ng has merged with k. The ḵ indicates that it corresponds to ng in other dialects. There is no difference in pronunciation between ḵ and k.
Note that the Unicode character names of precomposed characters whose decompositions contain U+0331 ◌̱ COMBINING MACRON BELOW use "WITH LINE BELOW" rather than "WITH MACRON BELOW". Thus, U+1E07 ḇ LATIN SMALL LETTER B WITH LINE BELOW decomposes to U+0062 b LATIN SMALL LETTER B and U+0331 ◌̱ COMBINING MACRON BELOW.4
The Vietnamese đồng currency sign resembles a lower case d with a stroke and macron below: U+20AB ₫ DONG SIGN but is neither a letter nor decomposable.5
"Combining Diacritical Marks Code Chart, Range: 0300–036F" (PDF). The Unicode Standard. Retrieved 2016-11-21. https://www.unicode.org/charts/PDF/U0300.pdf ↩
"6.2 General Punctuation" (PDF). The Unicode Standard. Version 11.0.0. Mountain View, CA: The Unicode Consortium. 2018. p. 273. ISBN 978-1-936213-19-1. Retrieved 2018-12-12. Spacing Overscores and Underscores. U+203E OVERLINE is the above-the-line counterpart to U+005F low line. It is a spacing character, not to be confused with U+0305 COMBINING OVERLINE. As with all overscores and underscores, a sequence of these characters should connect in an unbroken line. The overscoring characters also must be distinguished from U+0304 COMBINING MACRON, which does not connect horizontally in this way. 978-1-936213-19-1 ↩
"Latin Extended Additional Code Chart, Range: 1E00–1EFF" (PDF). The Unicode Standard. Retrieved 2016-11-21. https://www.unicode.org/charts/PDF/U1E00.pdf ↩
"Unicode character database". The Unicode Standard. Retrieved 2016-11-21. https://www.unicode.org ↩