Tough Creatures with unexpected magical aptitude.
These Creatures are tanks boasting high Health Growth and Armor in exchange for most of their other Stats. Strangely, it also has one of the highest Magic Growth Stats in the game. Despite their base Magic being really low, it can actually get quite high if raised well.
You could make use of that by giving them Telekinetic, fuse them with a spirit for a low cost spell like WaterBolt, or really make a mage out of them by giving them an ability to increase their Mana so they can use high cost spells. That last option isn't really worth your time and effort though, unless you really want a massive, magic turtle that can cast Tsunami to condemn its enemies to a watery doom.
The Land Mount ability gives them the option to carry smaller party members, reducing the time needed to traverse a land hex.
The Water Mount ability gives them the option to carry smaller party members, giving them the ability to traverse deep water and reducing the time needed to pass through water hexes.
If one breed/adapts the Heat Resistant and Floating genetic traits onto a large Turtle it can serve as a powerful Omni-mount, though the lack of Oxygen Provider means they'll have trouble taking your party into a Water Portal by themselves.
They can be sold for 100 gold at Medium size and for 300 gold at Enormous size. Every level above 1 additionally increases their price by 5%.
The portrait actually shows a tortoise, which have legs. Turtles have fins.
Turtles being mounts with magical potential might be a nod to various mythologies and other stories. Real turtles have no value as mounts, nor magical potential.