We know the Founders have a strong disdain towards the solids and it seems like the only solid life forms they tolerate are the animals as (in Laas words) they live the way they were meant to. As controlling and evil the founders were I always had an ounce of sympathy for their fight. Humanoid solids can be treacherous and I understood why she wanted order on the Alpha Quadrant but obviously she went overboard. All humanoid solids have a violent history but I'd say the cardassians and romulans really gave the solids a bad reputation especially when both species attempted to eradicate the founders. Which species do you think gave the solids a horrible reputation? I'm sure the founders would find this thread pointless as "Solids are solids" they are all inferior mono-forms incapable of shape shifting.
Humans? The only species that actually tried to slowly murder the Founders by making them suffer horribly from a degenerative disease, which they used a shapeshifter that trusted solids for (Odo) as a carrier- and keeping it a secret. At least the Cardies and Romulans were relatively upfront about it, and tried to bomb them by a surprise attack, nothing more. Seriously though. According to the statements of the female Founder, their distrust of the Solids grew way back in the past, and it (supposedly) eventually lead to the establishment of the Dominion. According to which Weyoun statement you go with, the Dominion is either two thousand or ten thousand years old. So it can't have been any of the Alpha Quadrant races. Also she says "The Great Link tells us that many years ago our people roamed the stars, searching out other races so we could add to our knowledge of the galaxy. We went in peace, but too often we were met with suspicion, hatred and violence." so probably it wasn't a single race, but many of them that made them distrust the Solids. That is, if you accept those statements at face value. We don't know if she is speaking the truth here, after all.
I wonder if Odo felt the way about Quark that he did because of his personality, or if there was a tinge of the Founders in there. Ferengi are treacherous and very rigid in their adherence to their economic beliefs. I know the Ferengi Alliance maintained neutrality during the Dominion War, but if the Dominion won, I don't see that lasting too long.
What if the precursors to the Jem'Hadar were the ones primarily responsible for persecuting the early Changelings, and in revenge, the Founders genetically engineered the entire species and turned them into slaves of the Dominion?
I would assert that the worst solids were in fact the Jem’hadar and only due to their associations with the founders. The episode where the underlying care of Odo’s development was uncovered, it illustrated a great fallacy in the changeling view of solids. Besides, genocidal proponents get a weak vote on oppression.
I doubt it was one particular species - probably just a general trend that races who came into contact with them treated them negatively because they were so different. Playing devil's advocate for a moment though, I'd suggest that being extremely wary of a race who can take on the appearance of anyone or anything isn't entirely baseless - the very nature of the Changelings makes them a massive threat should they wish to be (as they went on to demonstrate).
It definitely would have been a Gamma Quadrant Species. Maybe it wasn't any one species, but multiple ones. We do hear of multiple Gamma Quadrant species having unpleasant run-ins with or persecutions of Changelings in the distant past. Wasn't it the Hunter in "Captive Pursuit" who said something to the effect "a long time ago some Changelings showed up on my planet, we got rid of them"?
Whichever ones were most abusive to the Founders centuries ago, and the Founders ended up overthrowing and obliterating. Kor
Obviously, there's no right or wrong answer to this question, because it was never mentioned. Personally, I don't think, that was one species alone. It was an experience, they made over and over again at different planet, with different people and at different times. When there's some 'secret' species, the authors thought about, the only possible key, they could have given us, could be the casting of Salome Jens. I'm not sure if it was cool or cheesy, when it would have been the ancient humanoids. So we also would have been an explanation, why don't trust any humanoids: because most of the humanoids are descendants of the one species, they made 'bad experiences' with.
Less than one species and more the generalized behavior of solids. As Quark explains to Odo-they fear changelings because they are alien. Like their ancestors feared a snake in the grass or any other strange dangerous creature. Bipedal solid humanoids are all similar in physiology, if not culture. The founders are distinctly alien and thus instinct kicks in-they are feared and thus hated.
The species that gave solids a bad name, would be the Founders. Been bad mouthing solids for centuries.