The dog op is asking about is the Alano Español, sometimes referred to as the Spanish mastiff. Legend says the breed was originally brought to the Iberian peninsula by the Alans who settled there during the migration period (hence the name). Back in the day they were widely used as guard dogs, hunting dogs, bullfighting dogs, and war dogs. The breed isn't very common today and it's kind of dying out, as it's happening to many working dogs in developed countries that have less of a need for them.

While I can't identify the dogs, the text is very familiar, coming as it does from Sahagún's General History of the Things of New Spain, aka the Florentine Codex (I guess some people refer to this exact section as the "Messenger's Report?). The Codex often diverges into somewhat ornate and melodramatic language, so the description of the dogs is not particularly out of tone with the text.

The final book of the Codex though, from whence this section comes, does have a particularly over-dramatic feel. It is this book, that lays out the weak and quailing nature of Motecuhzoma, paving the way for the his capitulation to Cortes. The first half of the book is very much filled with ominous omens and terrified reactions on the part of the Aztecs, and Motecuhzoma in particular. In this particular section of the book, I think the Aztec leader is portrayed as openly weeping and even fainting from fear, so the description of the dogs as terrifying beasts, all slaver and jaws, is part of the tone.

Yet, the portrayal of the weak and cowardly Motecuhzoma has a number of flaws and the Florentine Codex, in particular, has been cited as propagating, if not originating, ideas of the Aztecs as crippled by the fear of omens and falling before a returned Quetzalcoatl. This post goes into some additional detail, but a major refutation is Townsend (2003) "Burying the White Gods," where she notes that the Christianized Nahua nobles who wrote and compiled much of the Florentine Codex had no actual first hand knowledge of the reaction to the supposed omens, but also had a vested interest in portraying the Conquest as divinely foretold, thus absolving them and their families from any guilt at the outcome.

Oh, and "spotted like an ocelot" can also be read as "spotted like a leopard," as ocelotl is the actual Nahua term for a jaguar. Ocelots were separately identified as tlacocelotl/tlacoocelotl. The decision to translate ocelotl as "ocelot" was a somewhat idiosyncratic decision on the part of Anderson and Dibble.