I read this a LONG time ago, so I don't remember much about it. I apologize in advance 😅

I know the MC had a dog that passed away, and so he picked up a white cat that reminded him of his white dog. But, I think the cat was actually tiger?? And he turns into a human man. But, I think there was a twist where the dog wasn't actually a dog, it was the cat?? And MC's memories had been altered? Again, I apologize, this is the most I can remember.

Could it be Tiger Lily? It’s been a while since I read it so I don’t remember everything but it has elements of what you described.

https://www.mangaupdates.com/series/odxfzlw/tiger-lily

No, it's not that one. Thank you for trying, though!