Of course they don't do it for every animal that dies, but I think it's a great way to pay respects of animals that have made a big impact on the zoo and it's history who warrant having their memories immortalised in a way. Like the few animals Colchester have done this for, Otto was not just another animal and was significant to the zoo by siring 6 of the 7 rhino calves to have ever been born at Colchester Zoo, over a period of less than 15 years. The other animals that this has been done for off the top of my head, White Rhino Flossy, Orangutan Rajang, Chimpanzee Billy-Joe, all lived at the zoo for many decades (since 1970s at least) and became favourites for staff and visitors alike, and in the case or Rajang reached a level of online fame. Billy-Joe has two daughters and two grandchildren in the current chimp group of five. As I said they don't do it for every animal and there is probably a handful or less living at the zoo now that this would happen more. For example in the last 5 years two other chimps (at the zoo since the 90s, one born there) and two tigers who arrived in the early 2000s have passed away without such treatment.