Probably some of the readers might have read in some Tibetan sources that “Ru” has been used as a kind of a title, say, in place of “Ācārya.” See, for examples, the Klong chen chos ’byung (Lhasa: Bod-yig-dpe-rnying-dpe-skrun-khang, 2013 [reprint of the first edition 1991], p. 318): Ru ’Jam-dpal-bshes-gnyen, Ru Padma, and so on. Note that “Ru” is also used with Tibetan authors, for example, Ru bKa’[= sKa]-brtsegs. When I first encountered such a usage, I was totally clueless. (a) Recently, however, Ms. Mengyan Li, one of my doctoral students who is writing her dissertation on the history of rDo-rje-phur-pa cycle of Tantric teachings in Tibet, suggested that “Ru” seems to be an abbreviation of “Guru.” This possibility did not occur to me and I think one should give Ms. Li the due credit for coming up with this idea. (b) Could “Ru” have been an abbreviation of Ru-dpon? It is true that a ru dpon or ru sna is military term and can mean something like “the head/leader of a regiment” and hence a kind of a military general. But possibly ru dpon may reflect a Tibetan equivalent of ācārya or a phase of the Tibetan attempt to make sense of the Sanskrit word ācārya, which then later came to be rendered into Tibetan as slob dpon. Can it be that, at least initially, Tibetans understood both ru dpon and slob dpon as some kind of a “guide,” “instructor,” or “trainer”? Incidentally a rectangular ruler used by traditional Bhutanese architects is called a slob dpon. A search in the OTDO reveals ru dpon but not slob dpon. Possibly also the term slob dpon was created (somewhat later) by Tibetans to render ācārya, and slob dpon seems to literally mean “an instructing or training leader/master.” (c) Dan Martin, however, has asked if there is any reason (see below), why “Ru” could not have been an abbreviation of “Rudra.” Initially, I have claimed that contextually this seems very unlikely and that it is very unlikely that Ācārya Mañjuśrīmitra would be titled “Rudra.” But now I am reconsidering this possibility. Mr. Nicola Barjeta, a student of mine, points out that according to MW (s.v. rudra), rudra is also a “name of various teachers and authors (also with ācārya, kavi, bhaṭṭa, śarman, sūri …).” It is, however, not quite clear to me what MW actually means here. It would be interesting for me only if rudra is interchangeable with ācārya, which does not seem to be what MW means. The PW has just “Nomen proprium verschiedener Männer.” The fact that “Rudra” can occur as a Nomen proprium seems to be of no relevance to the present question.
For fun, consider the following compounds: sde dpon, dmag dpon, khri dpon, mda’ dpon, khyim dpon, bza’ dpon, khrims dpon, skyor dpon, rdzong dpon, gsol dpon, gzim/gzims dpon, mchod dpon, gar dpon, ’go dpon, tsho dpon, mnyan dpon, grong dpon, ded dpon, lding dpon, sgar dpon, sger/sgos dpon, sgo dpon, brgya dpon, bcu dpon, chibs dpon, ’cham dpon, ja dpon, jag dpon, jus dpon, rje dpon, mgo dpon, gter dpon, drag dpon, drang dpon, nor dpon, yul dpon, tshong dpon, par dpon, las dpon, spyi dpon, phogs dpon, rtsis dpon, phru dpon, brang dpon, zong dpon, mdzo dpon, mdzod dpon, zhal dpon, zhi dpon, gsol dpon, gzhas dpon, bzo dpon, g.yos dpon, lag dpon, she dpon, sho dpon, so dpon, and so on. The list would be, by no means, complete.