My first 2 dogs were a couple of Black and Tan/Walker crosses given to me by a kid going on a Mormon Mission. They were both about 18 months old and had been on a couple of lion the previous winter. One had won the puppy heats at a field trial and the other one was a unatheletic, bow legged, pancaked footed, mutt looking hound rescued from the pound. To this day I've never owned an uglier dog. I got these dogs in 1982 when I was in my early twenties and hunted them hard. I never had anyone to show me the ropes so I learned everything the hard way. I think I only caught 3 lion with them that first winter. By the second season I started to notice that the mutt looking hound was starting to out do the field trialer, and by the third season she was catching every lion I dumped her on (she was still too unatheletic and slow to keep up with a jumped bobcat).
She was my lead dog until May of 1991 when I turned her loose on a travelling tom lion after a spring snow storm never to see her again. I tracked her on foot for over ten miles but the snow finally melted out and I was unable to continue (I never had tracking collars back in those days). I suspect she foundered up and died.
I've never lost a dog to a bear or a lion (knock on wood), but I've had a few roughed up and witnessed some of my buddy's dogs get killed. My dogs are primarily cat dogs and will usually keep their distance on a bear. My current lead dog won't even run a bear anymore after a bad experience near Salmon, Idaho a few years back. But that's okay, 90% of my hunting is done on cats and I always have some young dogs wanting to prove themselves. I ain't too worried about losing a dog to lions, but with bear it is a matter of time.