I'll throw in a vote for a game that is almost never mentioned. I don't actually think I've ever seen it discussed here.Web Earth Online.
It's an absolutely ancient java-based web game that is still suprisingly online, despite being virtually empty of players most days.
It's like of a persistent online ecosystem simulation. The world is populated mostly by NPCs. As the years cycle by, each season affects how the animals behave and survive. Birds build nests, rabbits use dens, etc. Players can play as any of the animals in the game (some are subscriber-only, however). It includes snakes, elk, wolves, condors, rabbits, etc.
If you can get used to the lag, it's a rather deep game, with a lot of interesting gameplay elements. Bigger creatures can 'carry' smaller ones back to feed their young (if they're still alive, they can escape if you drop them). Carcasses rot, and some animals only eat fresh meat, whereas others prefer carrion. The smaller bird species need to build their nest in a suitable tree by gathering materials.
To breed, you must find either an NPC or player of the opposite sex, and then females will give birth/lay eggs and the chicks must be fed. You can name your chicks (who become automated NPCs) and they'll keep that name for as long as they survive in the world. Chicks, by the way, are vulnerable to animals such as snakes, who can enter nests and Carry them away.
My favourite memory of this game was as a condor. I was a very big, strong female and had raised several clutches of chicks. I had reached the end of my lifespan and was stealing chicks away from a pair of player-controlled Hawks, who then hounded me across the world for my deeds. I managed to find an abandoned wolf den and hid inside, but they eventually found and killed me. I was minutes away from dying of my age and infested with parasites, anyway. xP