In attestations of the Mari language from the 18th-century, Mari /ŋ/ tends to be represented with the Cyrillic letter <н>. Lots of manuscripts represent MariE jeŋ ‘person’ as <ен>, for instance. For more examples, see Alhoniemi’s 1979 commentary on the Mari wordlist of P. S. Pallas.
A colleague of mine found this odd, as he would have expected the sequence <нг>. Yet, denoting the sound [ŋ] in the same way as another single consonant has a long history. Consider Greek where the sequence [ŋg] is always spelled <-γγ->. Also, a samoyedologist once told me of a foreign colleague (Japanese, if I recall correctly) who kept hearing Nenets /ŋ/ as /g/; his ears simply couldn’t pick up on the nasal property of the consonant.
But if historically /ŋ/ has been confused by other peoples as either /n/ or /g/, the question remains why these Russian (and Russia-resident German) wordlist compilers constantly denoted Mari /ŋ/ with the symbol for /n/ and never for /g/. One reason for this may be that the compilers were already using Cyrillic <г> to represent Mari /ɣ/, which is a fricative, not a stop. Since the only other voiced velar sound in the language was a fricative, the velar stop /ŋ/ was heard as the closest stop to it: /n/.
But in the neighbouring Udmurt language, where the /g/ is a stop, not a fricative, 18th-century compilers still denoted /ŋ/ with the same symbol for /n/. D. G. Messerschmidt’s wordlist, which has been reprinted with a commentary by V. V. Napolskikh, has <Gurpuhn> for Udmurt dial. gurpuŋ ‘heron, stork’. (Note, however, how Messerschmidt denotes the sequence [ŋg] in <Ning-goron> for Udmurt dial. ńiŋgoron ‘woman’.)
So what else in these Uralic languages and in the native languages of these Russian and German compilers could have motivated the choice of the letter usually denoting /n/ and not the letter for /g/? Something worth thinking about.