could be scifi or pure fantasy — i guess fantasy — remote jungle outpost where every human is turned into an animal. Harry is turned into a gorilla while at an even more remote camp. Tries to get back to the main camp; realizes he is being tracked by a large cat (leopard?). gets closer, realizes that there is a peculiar silence that means that there is another large predator near (lion?). Harry hopes the lion will keep the leopard away.
If it’s scifi, it’s a remote jungle planet; better, how about it’s part of the tree universe, this is a tree that’s been customized to have 19th century african jungle and other habitats.
Finds out lion is another changed human, hunting for food, but totally inexperienced at it.
Manages to communicate with the lion that he is also a changed human, can’t talk but can write in big block letters in the dirt, as can the lion, though even more awkwardly. (Lion female, possible romance?)
Discover other humans in other large animal forms — maybe some were small animals, as well, but those don’t (succeed/live?). No unchanged humans remain.
1) Humans retain full intelligence and personality, but only physical abilities of animals
2) become more animal like over time?
3) no motor skills training - hence writing is very difficult and requires great conscious effort.