You may be right. There can be only one star of a Lucy series, as it should be. "Horse Guest" is a pleasant enough entry. Memorable mainly for Frawley's cameo. You have to overlook a lot of plot holes. Such as: how did they get the horse into the Glenhall apartments since Lucy is on at least the 2nd floor if she's got that old crank Mrs. Golddapper living below her. Ann adds a lot to every episode she appeared in. Being a weekly regular probably wouldn't have worked out but I don't know why the 3-4 guest shots a year didn't continue. I think My Mother the Car was the next season but that didn't take much of her time. Speaking of MMTC: I recently bought a batch of Chicago TV Times from the mid-60s. The "Mailbag" column has some interesting, if dumb questions. Looking them over, I don't think they are supposed to be jokes. Someone wrote in:" In My Mother the Car, is Ann Sothern saying her lines from the trunk?". The answer only addresses the fact that this particular car doesn't have a trunk. Other questions "How does Samantha on Bewitched perform her magic?" "Does Mr. Ed actually speak the English language?" and "On the Patty Duke Show, are Patty & Cathy played by the same actress?" . Mailbag answer: yes. My answer: watch the opening credits, you moron!
Also notable about "Horse", Herb Vigran gets entrance applause which I'm sure pleased him.