The LT/MM bulldogs are all quite similar to one another, especially their 1950s characters. In the 1950s, we have Spike, Hector, and Marc Anthony. They all have a very similar appearance, but they have different names, and appear in different contexts. On that basis, people identify these as different characters, just as people distinguish the very similar Wile E. Coyote and Ralph Wolf, because they appear in different contexts, have different names, and in this case, different characters (mute vs talking) and slight differences in appearance (Ralph's nose is red, as I recall).
But as I work my way through the LT/MM filmography, it's interesting to find earlier instances of characters. So for example there are very similar bulldogs in Behind the Meatball and Birdy and the Beast. I very much expect that as I view older cartoons, I will find other bulldog characters - it seems to be a generic Termite Terrace stock character.
In fact, I would argue that the bulldog in Birdy and the Beast (called Butch) is an earlier appearance of Hector than 1945's Peck Up Your Troubles, which is listed in this wiki as being Hector's first appearance. That's true if you regard the different names as signifying different characters. From the perspective of character design, there are bulldog precursors in the LT/MM shorts that predate Hector, but are substantially similar. So the bulldog in Behind the Meatball, being unnamed, could very well be an earlier appearance of Hector.
It's interesting to watch the cartoons, and to identify precursors to the star characters. The canary in The Cagey Canary predates Tweety's first appearance in A Tale of Two Kitties by over a year, but is clearly very similar to Tweety (a big difference being the voice.) The structure of that cartoon very much follows that of the later Tweety/Sylvester shorts too.
I can even point to an early prototype for Pepe Le Pew, namely Killer Diller the Ram in I got Plenty of Mutton (1944) (not to be confused with the gangster Killer Diller from Thugs with Dirty Mugs). This ram has much of the character and behaviour of Pepe Le Pew a year before the first identified appearance of a Pepe-like skunk in Odor-able Kitty. Because Pepe has been reported to have been inpired in different ways, the existence of Killer Diller as a relentless French romancer may give clues as to the actual inspiration. Note that I got Plenty of Mutton was directed by Frank Tashlin, so perhaps he should be considered one of the creators of Pepe as well.
What do you think about these things? Did Hector make earlier appearances? Is Killer Diller Pepe in disguise? I look forward to reading your comments, and perhaps we can put what we discover into the wiki pages.