"Pal (dog actor)" on @Wikipedia:
"By 1957, Pal was growing blind, deaf, and stiff, and rarely visited the Lassie set. The star of the show Jon Provost later recalled, "As young as I was, I recognized how much that dog meant to Rudd. Rudd loved that old dog as much as anyone could love an animal or person."
Pal died of natural causes in June 1958 at age 18 (about 85 in "dog years"); and for months Weatherwax slipped in and out of deep depression. Robert Weatherwax, Rudd’s son, later recalled, "It hit him very hard when Pal died. He buried him in a special place on the ranch and would often visit the grave. Dad would never again watch an MGM Lassie movie. He just couldn’t bear to see Pal. He didn’t want to have to be reminded of just how much he loved that dog."