Tosfos offers a second answer:

וה"ר מנחם פי' דה"ק ‘מועד לאדם‘ - שהיה מועד לכל וחזר בו מבהמה ונשאר מועד לאדם ‘הוי מועד לבהמה‘ - דחזרה דבהמה לאו חזרה היא.

Answer #2: Whereas R. Menachem explains the Gemara as follows: 'Mu'ad le'Adam' (that was initially Mu'ad for everything and retracted from Beheimah but remained Mu'ad for Adam) is (still) Mu'ad li'Beheimah', since retraction from animals only is not considered a retraction.

אבל מועד לכל וחזרה מאדם לא הוי מועד לאדם, דחזרה דאדם חזרה היא.

But if it is 'Mu'ad for Beheimah (which was initially Mu'ad for everything, and retracted from Adam) it is not (anymore) Mu'ad for Adam', because retraction from Adam only is considered a retraction.

This Rabbeinu Menachem requires understanding!! Logically, the opposite of what he said makes sense. If the ox only gored people and we have no proof that it doesn't gore animals, nevertheless we don't say [according to Rav Pappa] that if the ox was מועד לאדם who has מזל [making him harder to gore] it is also מועד לבהמה. [We view the relationship to animals independently and since it doesn't gore animals, it is not a מועד לבהמה]. Certainly then, if the animal was מועד לכל and retracted from goring animals [so we see that it doesn't gore animals] we shouldn't say that since it remains מועד לאדם who has מזל, certainly it is מועד לבהמה and that the retraction of the animals is not a retraction. [Rather we should view the relationship to animals independently and say that since the ox stopped goring animals, it is no longer a מועד לבהמה].   

Rav Abba ztz"l explains as follows: There are 2 aspects to the דין מועד. One is that through 3 gorings the animal is considered to be בחזקת נגחן - a habitual gorer - as the pasuk says "או נודע כי שור נגח הוא". The second is that the very gorings are effective to give the animal a "דין מועד" with respect to נזק שלם. This is clear from the fact that Tosfos writes later [כ"ד, ב ד"ה במכירין בעל השור] that an animal only becomes a מועד when the gorings create a monetary obligation. But if the animal gored a ownerless animal three times where there is no monetary obligation, it doesn't become a מועד. Why not? It gored three times? The answer is that we also require that the animal should have a "דין מועד". In order to receive this status, the animal must gore 3 times in such a way that incurs a monetary obligation.   

According to this it would seem that R' Menachem holds that even Rav Pappa agrees that with respect to the element of הוחזק נגחן, becoming a מועד for a person also makes the ox a נגחן for animals. This makes logical sense - if the ox is a מועד for people who HAVE מזל [thus making goring them more difficult], certainly it is a מועד for animals who don't have מזל. Just with respect to the second aspect, the "דין מועד", Rav Pappa holds that מועד לאדם is not מועד לבהמות. Because he holds that it is not enough for the ox to be considered a "חפצא of מועד" alone, but rather it needs to be a מועד for the damaged party. Therefore, since there is a distinction between animals and people [especially according to the way we explained the opinion of the רמ"ה - that אדם needs a stronger נגיחה than a בהמה, and R' Menachem apparently agrees with this which is why he didn't accept Tosfos' first answer], when the ox becomes a מועד לאדם it doesn't follow that it will also become a מועד לבהמה. However, when the ox becomes a מועד for both אדם and בהמה and then retracted from being a מועד לבהמה [by not goring animals when it had the opportunity to do so], since it remained a מועד לאדם [in other words it was הוחזק נגחן לאדם and automatically it is still הוחזק נגחן לבהמה], therefore the original דין that it was a מועד לבהמה doesn't go away. That is what R' Menachem meant when he said that retracting for the בהמה is not effective since it remains a מועד לאדם [i.e. הוחזק נגחן] it certainly remains a מועד לבהמה so the retraction from goring animals doesn't change anything.  

Wonder of wonders!!😊😊