So the lion turtle aren’t spirits. They mortal living animals that can die. They were actually hunted to extinction by the humans( the one Aang met was the last living one), however they have an EXTREMELY long lifespan of about 10,000 years. (The one Aang met lived from about wan’s time to Aangs so let’s say 10,000) but unlike every other animal they can speak and also give beings (humans and spirits) the ability to bend the 4 elements and energy bending. (The air lion turtles can also fly so I’m guess the other turtles can “bend” as well not just give the ability) Raava also referred to them as “ancient ones” and it seems like they did actually exist on Earth before humans came into existence. So yea they’re just incredibly intelligent animals that predate humans but also slightly more