Old men of the river

"Legend has it that one day the Valyrians netted and butchered one of the giant turtles the Rhoynar held sacred."

- Varys

Giant Turtles are very large reptiles that live in the Rhoyne River, the largest river in the Free Cities, and its network of tributaries in Essos.

The Rhoynar people revered them as sacred.

In the books
In the A Song of Ice and Fire novels, the Rhoynar called the giant river turtles a name which translates as "Old Men of the River". They were considered sacred in the Rhoynar religion, as one of the consorts of their primary deity Mother Rhoyne (the goddess of the river) was a minor deity that took the form of one of these giant turtles, known as the "Old Man of the River".

The giant river turtles are described as "enormous": far bigger than say a direwolf is relative to a common wolf. Despite their size, encounters with them are very rare: they are possibly few in number and certainly reclusive, as they seem to spend most of their time submerged underwater.

When Tyrion Lannister travels from Pentos to Volantis in the novels, he does so by riverboat down the Rhoyne (Season 5 of the TV series condensed this to just a long journey over land in a large horse-drawn wheelhouse). As they make their way down the Rhoyne past the ruins of the Rhoynar city Ny Sar, one of the giant turtles briefly surfaces near the boat, then disappears again. Tyrion judges that it was as big as the boat he was traveling on, the Shy Maid - which wasn't just a raft or fishing boat, but a one-mast poleboat large enough to have three internal cabins (though this was still described as a modestly sized boat for long-distance travel). Based on his description it would seem that the giant turtles can grow to roughly the size of adult elephants.