What breeds of dog are Snoopy, Odie, Rantanplan, Dogmatix/Idefix,
Lassie and any other celebrity or pop culture dogs from movies, comics
or TV? Sometimes the breed is clear, other times it has to be figured
out from the dog's features.

This is one of the best sites I've found for miscellaneous information
about celebrity dogs, both actual and fictional:

Index of Famous Dogs
http://www.citizenlunchbox.com/famous/dogs.html

Where the breed of the dog is known, the site will mention the name of
the breed, as in the cases of Snoopy (a beagle) and Lassie (a collie).
Odie, on the other hand, is described as "Garfield's misshapen dog
friend." Rantanplan and Dogmatix/Idéfix are mentioned in the index.
But, like Odie, they are too stylized to be assigned a breed with any
certainty. These are just personal guesses from a dog-lover: I would
call Odie a beagle, Rantanplan a great Dane, and Dogmatix a
wire-haired fox terrier.

The Index of Famous Dogs is a part of this larger site, which includes
information about famous cats of fact and fiction, plus miscellaneous
"critters" who are neither canine nor feline:

Indexes of Famous Dogs, Cats, and Critters
http://www.citizenlunchbox.com/famous/animals.html

I hope you find the Index of Famous Dogs useful! If anything I've said
is unclear, please request clarification; I'll be glad to offer
further assistance before you rate my answer.

Best wishes,
pinkfreud