vaca.

Tla xiquimittaca[n] yn xallatauhca yn achto otlaneltocaque yn do[n] Alonso yeh icapa ye ysonbrero mochiuhque yn ipilhua[n] yhuan in teyacanque omochmocuapque[ue] ye moch quaquaque aocmo altepeneçi yn o[n]can onoque ça ixtlahuaca[n] quauhtla y[n] ce[n]mantinemi vacastin. (Anales de Juan Bautista, f. 8v) = Just look at the people of Xallatlauhco who were the first to convert. The children of Don Alonso turned into his cape and sombrero, and all the leaders there were transformed and turned into grazing cattle. No longer is the altepetl recognizable, and those who are there now live only in empty land, in forests, where cows have taken over.
Ezekiel G. Stear, Nahua Horizons: Writing, Persuasion, and Futurities in Colonial Mexico (Tucson: University of Arizona Press, 2025), 126–127.

In many of the documents in the corpus, a cow is called a vaca (or baca, or vaquilla, etc.), whereas huacax, originally the same word, is often an ox. Thus possibly the reference is to an ox in this case. (Santa Ana, Toluca Valley, 1737)
Caterina Pizzigoni, ed., Testaments of Toluca (Stanford: Stanford University Press and UCLA Latin American Center Publications, 2007), 119.

çe huacaz onicmacac yhuan çe escaRama = I gave him an ox with an escaramán; çe yonta hu[a]caxti = a yoke of oxen. Today an escaramán is a kind of large, heavy harrow pulled by oxen and used in preparing the soil for sowing. (San Miguel Aticpac, Toluca Valley, 1711)
Caterina Pizzigoni, ed., Testaments of Toluca (Stanford: Stanford University Press and UCLA Latin American Center Publications, 2007), 72.

huacax (well attested in Toluca)
Caterina Pizzigoni, ed., Testaments of Toluca (Stanford: Stanford University Press and UCLA Latin American Center Publications, 2007), 47.

Sentel baca = a cow (San Pablo Tepemaxalco, Toluca Valley, 1681)
Caterina Pizzigoni, ed., Testaments of Toluca (Stanford: Stanford University Press and UCLA Latin American Center Publications, 2007), 149.

oc nome baCas = two other cows (Calimaya, Toluca Valley, 1762)
Caterina Pizzigoni, ed., Testaments of Toluca (Stanford: Stanford University Press and UCLA Latin American Center Publications, 2007), 223.

centzontli yhuan zenpoali onmactlactli tzontecomatl bacas toros becerros be[ce]rras gueguey yhuan tepitzitzin = cuatrocientas y treinta cabezas de ganado mayor, vacas, toros, becerras y becerros grande y chico (Tepexi de la Seda, 1621)
Vidas y bienes olvidados: Testamentos en náhuatl y castellano del siglo XVII, vol. 3, Teresa Rojas Rabiela, et al, eds. (México: CIESAS, 2002), 110–111.

bacas, bacastin; quaquahueque baqastin (Cuernavaca, s. XVI)
Brígida von Mentz, “Cambio social y cambio lingüistico. El ‘náhuatl cotidiano’, el de ‘doctrina’ y el de ‘escribanía’ en Cuauhnáhuac entre 1540 y 1671,” in Visiones del encuentro de dos mundos en América: lengua, cultura, traducción y transculturación, eds. Karen Dakin, Mercedes Montes de Oca, Claudia Parodi (México: Universidad Nacional Autónoma de México, 2009), 120, 122.

ce[n]mantinemi y vacastin = por todos lados andan las vacas (ca. 1582, México)
Luis Reyes García, ¿Como te confundes? ¿Acaso no somos conquistados? Anales de Juan Bautista (Mexico: Centro de Investigaciones y Estudios Superiores en Antropología Social, Biblioteca Lorenzo Boturini Insigne y Nacional Basílica de Guadalupe, 2001), 158–159.