Slender Dog! Slenderman's Puppy!!

Anyway, Slender Dog is very dear to Slenderman.

Because it's not just a house-keeping dog.

If Slenderman's children run away, Slenderdog can be tracked using 

a sense of smell 10 times better than normal dogs.

Also, Slender Dogg can run at speeds of 100 kilometers per hour. 
You can't even dream of running away from Slender Dog