As always, it’s of vital importance to forge an understanding of your pokémon before proceeding. It’s entirely possible that your lycanroc’s behavior can be explained by his evolution and thus shift in personality. Midday lycanroc are indeed highly loyal pokémon, but they’re only loyal to those they perceive as being members of their pack. To anyone outside of that pack (as the cat-like torracat would be), they may become defensive or even territorial.
Alternatively, your lycanroc may be experiencing the beginnings of his maturity. That is to say, seeing as lycanroc and torracat are compatible with one another in terms of breeding, it’s possible that your lycanroc is attempting to drive away what he perceives to be an inadequate breeding partner, or he may be dealing with conflicting feelings about his former best friend.
On the other hand, it may also have nothing to do with your lycanroc’s evolution at all, and it may simply be a coincidence. It may be that your lycanroc and torracat are going through a personal quarrel or a misunderstanding that has less to do with your lycanroc’s evolution and more to do with how much food they receive, what toys they have, or something entirely different.
Thus, the first step is observation and communication. Observe your pokémon to see if you can determine a point of conflict, and if you can’t, simply ask them why they’ve started fighting. If you can determine the root cause of your pokémon’s arguments, you may be able to work from there to settle things once and for all.
Best of luck, anonymous!