This confused me so I asked my wife who is a native speaker. She says "מסמן את הסוס " is to put a mark on the the horse (e.g. Draw an X on it). But " מסמן לסוס " is to signal to the horse, like to tell a trained horse to do something by waving your hand. I hope that's right - she rarely lies to me.
Yes, I had gotten the horse sentence first, so when the dog sentence came up I was confused, thinking "mark" didn't make sense there. Then, when I looked it up and saw that it could also mean "signal", I decided to try that in my answer the next time the horse sentence came up, and it was accepted. But, according to jrwilco1's wife, it sounds like "the man signals the tall horse" SHOULDN'T be accepted here because it DOES have את. It that right?
I heard an interesting tidbit about why an X can be used for the signature of an illiterate person. In Job 31:35, Job says, “Here is my signature!” In Hebrew, he says, “Here is my tav” (or taw), the last letter of the Hebrew alphabet. In modern Hebrew, tav is ת, but in Paleo Hebrew, it’s X.