Baby poop question

So my son is going to be 5 months on the 9th next month and just went for a check up about 3 weeks ago and he is healthy but I've noticed in his stool that there's always what looks like to me black strand. We have a cat and a dog so hair is everywhere but try to not get it on his things but it's pretty impossible not to lol. Do you think its hair that he accidentally swallowed or something else. He is formula fed. We are about to start giving him rice cereal. Picture down below of what's in his poop.