Correct spelling for the English word "ultroneus" is [ˈʌltɹə͡ʊnɪəs], [ˈʌltɹəʊnɪəs], [ˈʌ_l_t_ɹ_əʊ_n_ɪ__ə_s] (IPA phonetic alphabet).
Of one's own accord; voluntary.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.