Playing an animal… I once ran a homebrew science fantasy campaign (I called it Horse Chosen) where the PCs all had telepathic Companion Animals (horses, obviously). The session where all the humans had been drugged unconscious and the horses had to solve the plot was very entertaining to run. The players were very inventive at coming up with ways for horses without opposable thumbs could get things done.
I think a campaign of animals being portrayed ‘realistically’ would probably run into the same problems that running a proto-caveman/apeman game does. Namely, if you are running a culture with no recognisable economy, politics, job roles, and so on, you very quickly run out of things for the PCs to do. Run away from angry mammoth, defend camp from hungry sabretooth, go bison hunting, gather nuts and berries. Rinse and repeat.
Also, as zoologist, I point out each individual animal has ALL the survival skills necessary to, er, survive. So all the characters will be very similar, unless you are doing some sort of multi-species Jungle Book or Animals of Farthing Wood game, which drifts towards anthropomorphism and fantasy tropes. It’s not like real world wolf packs have warrior wolves and hunter wolves and primary school teacher wolves and accountant wolves…
The exception being those species of ants where they have caste dimorphism: where some are big soldier ants, others small worker ants, etc. It wouldn’t be much fun for the player who rolled up the winged drone ant or the honeypot ant! Much as it wouldn’t be fun to play Rogue Trooper’s rucksac or helmet, if and when Worlds of 2000AD does a Rogue Trooper sourcebook for their RPG!
I bought the Blue Rose RPG because I wanted to see how they handled sentient animals as a character type. Those are Narnia type fantasy animals, though they are telepathic rather than talk out loud. I’ve also played in a one-off game of Mouseguard, but the mice have swords, etc.
For playing a sentient but dim machine, I’d recommend the Cthulhu Hack scenario Valkyrie Nine, where the PCs are all maintenance robots on a lunar base, wondering where all the humans have gone. And David Wake’s novel, I, Phone where a smart phone has to try and prevent its owner being murdered.
On VampireTM and playing a monster… I like actions to have consequences in my games. If you kill people the cops are going to be running a murder hunt, regardless whether you are an undead creature of the night or a totally human mugger with a knife.