The idea is, in short, centered around two different types of creature that ultimately comprise a single keter(?)-class SCP (hereafter referred to as X-1 and X-2). They’re not actually dogs, for the record. It's just a point of comparison.
A thing I noticed going through this idea is that "keter" doesn't necessarily equate to dangerous. Keter is a classification of difficulty in containing a thing. A blob of innocent jelly that keeps teleporting around Site-19 could be perfectly harmless and annoying, but containing it could be immensely difficult. A visual-hazard coin that murders anyone that looks at it, but can't travel on its own is quite dangerous, but if locking it in a box is all that is required to contain it, it could be classified as "Safe". Make sense?
Something like the ACS guide might be useful in giving a more in-depth look at classifications for you.
X-1 is discovered first; X-2, while anatomically quite similar, is understood to be a separate species entirely. The first X-1 is contained after there's already plenty of proof of the sort of devastation it can cause. It already has a sizable body count, so a heavily-armed task force is sent to retrieve it… only for it to follow them home willingly because it wants their rations. It initially reacts with hostility when approached, but over time (and feedings), it seems to become much more amicable towards foundation personnel, even beginning to become excited upon seeing specific staff members. Unfortunately, play and affection from a keter-class SCP are still incredibly dangerous.
I think it's important to point out that you need to be careful not to rely too heavily on the DOOMPUPPY aspect of this. If it's just a horrible eldritch beastie that wrecks shit, that's not all that interesting. You've got a decent overall idea here, and I don't think that focusing too much on the awful shit it can do will serve the story you're trying to tell.
As more instances of X-1 begin appearing (not all of which can be retrieved quite so safely), foundation staff begin experimenting with those that have responded best to captivity, attempting to gauge their intelligence to help with containment efforts. X-1 are found to be incredibly intelligent, roughly comparable to dolphins, and incredibly trainable; as well, they display a protective nature towards specific individuals they favor (they’re generally agreed to have “dog-like” behavior, hence the title). Still dangerous, just noteworthy.
Veil breaking is fun, but it's been done in a lot of ways. A big world-spanning scope sounds like it could be interesting, but I'd advise narrowing it down a bit during your first foray into skip writing. It's easy for big ideas to steamroll into things with lots of consequences, and without a really firm foundation in what the SCP-verse is all about, you could be writing endless paragraphs on stuff that's already been covered. How about instead of a bunch of these guys just popping up all over the place, how about a few popping up near the initial one? Focus on a "pack" mentality, perhaps? They run together, so they manifest nearby?
The appearance of type X-2 is when things start falling into place. The notable difference between the types is that X-2, while displaying similar behaviors, acts much more akin to a "wild animal", so to speak. X-2 are vastly more difficult to recover alive due to immense hostility, much greater size, and more concentrated toxic secretions (in low doses, secretions have an antidepressant quality, but even X-1’s secretions are too potent for humans to endure for long). Anatomical comparison to X-1 brings about the theory that X-2 is, essentially, the wolf to X-1’s domestic dog.
What about having the X-1/x-2 relationship being more of a predator/prey relationship? You can still keep the dog/wolf concept, but having the X-2s hunting the X-1s could bring some really good action to the story and also keep the scope relatively narrow.
The X-1s are all contained at Site-<wherever>, and they're struggling with containment of all the X-1 iterations popping up. Oooh, scary dangerous eldritch hounds! Fun! Then, they start finding X-1 corpses that are partially eaten. WTF could eat one of these things? Boom, X-2 shows up all murder-y like.
Further contributing to this theory, what had been thought to simply be naturally-occurring markings on X-1, like the unique spot patterns of leopards, are discovered to be both artificial and a previously undiscovered form of writing, identification left by something unknown. X-1 instances with writing on them begin breaching containment because of an outside, unseen influence, one thing about them becomes more and more apparent:
I'm following the trail on this so far.
These creatures belong to something else. Deadly as they are to humans (even when unintended), whatever domesticated them seems to view these entities as about as threatening as we’d view a housecat. The underlying horror of the Cosmic Hounds is the implication of how terrible their masters must be if their pets are so difficult for humans to control and contain.
I see this last bit as a security locked report to the 05 council or something, a potential for a K-class scenario or something. "Hey, let's keep an eye out for something truly awful that could come looking for its minions" or some such.
