SCP-XXX is a rat. It will track down the closest pet of a child, and approaches the pet when it thinks no humans are watching (thought it can be fooled by things like hidden cameras); the pet will just observe the rat, without attacking/barking/etc. The rat will then stand on its hind legs and gesticulate with its front paws, pantomiming taking away or getting rid of a human. Once the rat is done, the pet will give some gesture of agreement (even if doing so should be anatomically impossible for a member of that species). At most a day later, the rat will come back, put the child into a trance, and lead him/her away without anyone else noticing, somehow bypassing any and all locks and security measures.
Several days later the rat will come back to the pet and pantomime a demand for something; the pet will refuse with a negative gesture. The rat will leave and soon come back leading the child, still in a trance, who is now SCP-XXX-1. SCP-XXX-1 will ignore everyone around them, and will struggle, kick and bite if anyone tries to stop it. It will go straight to the pet and rip it apart with it's bare hands (even if the child normally wouldn't have the strength to do so). Once this is done the child wakes from the trance, with no memory of anything that happened over the past several days, and is traumatized by the bloody chunks of former pet in his/her hands.