Three examples I have just now come across (edit make that four examples - see "owl" below):
Lucius, having been turned into a donkey tries to draw attention to his plight, by calling upon the name of Caesar:
Et “O” quidem tantum disertum ac validum clamitavi, reliquum autem
Caesaris nomen enuntiare non potui.
And indeed I shouted “O” by itself eloquently and vigorously, but
I could not pronounce the rest of Caesar’s name.
Apuleius, Metamorphoses, 3.29
Later (7.3), wanting to clear his name for some crime committed while still a human, he tries to call out again:
... volui dicere: “Non feci.” Et verbum quidem praecedens semel ac
saepius immodice clamitavi, sequens vero nullo pacto disserere potui,
sed in prima remansi voce et identidem boavi “Non non” ...
I wanted to say: "I didn't do it." And although I shouted the first
word over and over again, without restraint, I simply could not
pronounce the second word, but I was stuck on that first word and
brayed again and again, “Non . . . non . . ."
Which I can't help pronouncing the French, nasally way, to imitate what I (an English speaker) think a donkey sounds like.
... cum dicit - ‘r’; non multum est hoc cacosyntheton atque canina si lingua dico; nihil ad me; nomen enim illi est.
... when he says - "r"; it does not make much difference if I speak this
in an incorrect connection of words and in dog-language; it's nothing to me; because
that's its name.
Lucilius, Satires, 9:389–9
To be honest, I'm not sure what this is about but clearly "rrrrr" sounds like a dog to the writer (an angry one, I imagine!).
Varro gives the following onomatopoeic etymology for the "night owl", noctua:
quod noctu canit et vigilat
because it sings 'noctu' ('at night') and stays up overnight
On the Latin Language, 5.11.76