I forgot Liz the Vet went back that far as a character. Also, I remember this thread the first time around too. 
I don’t know how or why but I’ve seen this thread before.
I recall it for one of the greatest lines ever used on a bulletin board “Steaming cup of Fido Jizz”
Liz the Vet has been around for ever. What’s changed is that eventually she and Jon started dating semi-regularly - she used to shoot him down incessantly, and while they once had a one-off date and it seemed to go well enough, they were never a regular thing.
Incidentally, I don’t believe any dog jizzes enough to fill anything that would look like a coffee cup. Medicine or urine sample, I could believe.
Jiz o pete, that’s one odd Garfield strip.
Update!
Buzzfeed namechecks this thread in an article here -
In which they hunt down Jim Davis and make him explain himself. Here’s what he has to say:
I’m sure we will sleep better knowing this.
This just needs to be quoted again, for the newbies and third page readers. Also, the link still works, so, hooray! 
I shared the comic on Facebook and one of my friends pointed out that it could be interpreted as the vet calling Jon a female dog in a roundabout way. In other words, it’s just a joke about her calling him a bitch for stealing her coffee.
The fourth panel has Jon asking for a refill.
There is no way someone Jim Davis’s age would use “canon” as an adjective. Try harder, edgy millenial.