Usually an old mare wearing a bell. The mare part is important, because in Spanish the word usually means a godmother (as opposed to simple madre, mother). Madrinas of the mulish variety are mostly seen under that name in South America, and if you tried to use that meaning in Spain, you'd get looked at funny. Weeding out the godmother meaning isn't easy, but it looks like most uses are in travel writing or novels set in South America, so it could be argued that it hasn't been entirely nativized. I can, at least, give you a usage example by Charles Darwin from his H.M.S. Beagle journal:
In the morning we found some thief had stolen one of our mules, and the bell of the madrina.