that's a comically sized leopard
there a plenty of dog breeds that are bigger than any male leopard
Male leopards can get up to 80-90 kg, so "plenty" is a stretch, and no dog whatever the size has a chance in a fight against a leopard. I agree that this one was comically sized though.
Leopards, jaguars, lions and tigers are all closely enough related to breed with each other, and hybrids can grow really huge, so a leopard/ siberian tiger hybrid maybe? Or just a manga leopard, it looks like a normal leopard apart from the size.
If society suddenly collapses, there will probably be quite a few rich people's pets around, and many in places they don't belong like Pablo Escobar's cocaine hippos. There will be crossbreeding and new species/subspecies.
A couple of years ago I read about the examination of the remains of a cave bear. The researchers found out that the huge bear was killed right there in its cave by a man with a spear (I wouldn't mess with that guy). Maybe he would have a chance against a big cat with stone tools, but these guys will need some pretty thick plot armour.