Implicating mastiff → domestic_dog
Link to implication

Reason:

Similar to terrier tag implying to domestic_dog.

We have five *Mastiff tags at e621:
1) 172 mastiff species
2) 117 tibetan_mastiff species
3) 4 bullmastiff species
4) 1 german_mastiff species
5) 1 brazilan_mastiff species
(NONE with an e621 wiki)

And similar to various *_terrier tags implicated to terrier:
suggest tags bullmastiff and tibetan_mastiff both imply to mastiff. (see below)

re tag #2: Tibetan_Mastiff : "is a large Tibetan dog breed ... in the mastiff family." (quote from top of https://en.wikipedia.org/wiki/Tibetan_Mastiff )

re tag #3: Bullmastiff : https://en.wikipedia.org/wiki/Bullmastiff says "a large-sized breed" "Molossian: Mastiff type"

No rush on other two tags (german_mastiff and brazilan_mastiff) since they are both empty. (see below)

re tag #4: german_mastiff tag currently empty (single post deleted), but probably should be aliased to Great_Dane. Great Dane article at Wikipedia says "Other names ... German Mastiff"

re tag #5: brazilan_mastiff, single post was deleted. "brazilan" seems to be a typo of "Brazilian".
https://en.wikipedia.org/wiki/Fila_Brasileiro: "Fila Brasileiro ... or Brazilian Mastiff"

* * *
(you can probably ignore my nitpicking blather below)

https://en.wikipedia.org/wiki/Mastiff says some good things.
Article also says some things that I think we could partially-ignore such as:

1) including Bulldog as a type of mastiff (doesn't make sense. Wikipedia's Mastiff article starts with "Mastiff-type means a large molosser dog.", but Wikipedia's Bulldog article starts with "The Bulldog, ... is a medium-sized breed of dog" (medium is NOT large).

2) "The descriptive term, mastiff-type, should not be confused with the breed, the English Mastiff, that is commonly referred to as the Mastiff."
English Mastiff being also known as "the Mastiff" seems to me Euro-centric, as there are other *Mastiff breeds:

A) Bullmastiff : https://en.wikipedia.org/wiki/Bullmastiff

B) Neapolitan Mastiff : https://en.wikipedia.org/wiki/Neapolitan_Mastiff

C) Spanish Mastiff : https://en.wikipedia.org/wiki/Spanish_Mastiff

D) Tibetan Mastiff : https://en.wikipedia.org/wiki/Tibetan_Mastiff

plus other breeds that are also known as fill-in-the-blank mastiff, such as:
Aksaray Malaklisi dog, "also known as the Turkish mastiff" (quote from top of https://en.wikipedia.org/wiki/Aksaray_Malaklisi_dog )