Greek has the medium participle ending in -menos. It has a couple of occurrences in Latin, too, of which I only seem to remember alere > alumnus now. How many words are there in Latin that contain this element? What is their origin? Are they borrowed from Greek, or was the participle or derivative suffix -m(e)nus ever productive in Latin?
Weiss cites, besides alumnus, femina (from a root meaning 'give suck'), calumnia (derived from *kalwo-mno-, from the root of calvor 'deceive'), and possibly also columna and the divine name Vortumnus (if related to verto 'turn', though it may rather be Etruscan). These are not Greek loans but native formations; I don't know when precisely they were formed, but the suffix doesn't seem to have been productive in Latin as we know it.
It seems that some people have proposed that -menos developed in a few Latin words to -mens rather than -mnus, but there is no consensus in favor of this etymology. (And it certainly doesn't seem to have been productive.) At least one current Wiktionary entry gives an etymology based on this idea: the adjective clēmens, according to Wiktionary, comes "from clīnō + participial suffix -menos".
De Vaan (2008) says an etymology from *klejomenos was given in Walde-Hoffmann (1930-1954), and earlier by Breal and Osthoff (the "nt" in non-nominative forms would have to be analogical). However, he says "the contraction in initial syllable of *ejo > ē is doubtful, and the disappearance of o-stem inflection completely unexpected." De Vaan thinks that a derivation from mēns 'mind', as given by Ernout-Meillet (1979), is "attractive". De Vaan also provides an alternative proposal for exact form of the first part of the word to try to make the ē a bit more explainable, *kloi-wo-ment-.
"Latin Etymologies" by Edwin W. Fay (1903), seems to say that Niedermann proposed deriving vehemens from *vehemenos, and mentions that Osthoff connected clēmens to Sanskrit "çrayamānas 'leaning' " (Note: this seems to correspond to the IAST spelling śrayamāṇaḥ/śrayamāṇas.) However, Fay says that clēmens and vehemens~vēmens are basically the only evidence for -mens as reflex in Latin of -menos, and therefore Fay rejects that etymology and instead proposes that both words are originally compounds from mens "mind". Fay also rejects the identification of the first part of clēmens with clīnō (he finds the semantic relationship underwhelming), proposing that it instead comes from a form *tlē- that he associates with Greek talai-phrōn and tlē-thymos; he suggests the development of the tl to cl instead of l is due to influence from forms prefixed with in-, specifically inclēmenter.
Osthoff's proposed etymology for clēmens also seems to have been mentioned by Brugmann (1891) alongside discussion of -mnus and -mini.
To clarify, the point of this post is not to endorse this etymology (nor deny it: I don't have the authority to do either!); I simply wanted to bring it up so that readers would be aware of the historical fact that in the past, some scholars suggested that -mens in these two words was derived from *-menus.
"Latin Etymologies", Edwin W. Fay. The American Journal of Philology Vol. 24, No. 1 (1903), pp. 62-74
"Comparative Grammar of the Indo-Germanic Languages" Volume 2, Karl Brugmann. 1891.
Etymological Dictionary of Latin and the other Italic Languages, Michiel de Vaan. 2008.