Giant turtles with dragon heads(Adults have frost breath)
You can't tame the adult ones that spawn naturally so you have to steal the eggs from the nest. The nests spawn on beaches in the black forest. Each nest has a chance to have 1-2 turtle eggs or gold/treasure.
When eggs are acquired, the nest building is available, costing fine wood, resin and deer hide.
When nest is placed, a maximum of 2 eggs can be placed in the nest. The egg must have a source of heat nearby and covered from weather.
Eggs take 5 in game days to hatch where they feed on raw fish, boar meat, neck tails and berries.
A turtle takes 3 days to grow into an adult, where a saddle can be crafted with Iron, Iron nails,fine wood and troll hide.
Turtles can move on sea and land, but are slow on land(a bit faster than trolls)
When they become adults, a hut must be made or the turtle will get aggressive and demolish its surroundings(hut made from core wood and bronze nails after unlocking nest).
Turtles have no storage, but when on land, they can pull carts.
They work kind of like dogs, but instead of following, they either stay where they are, or wander.
While wandering, if a hut is nearby, the turtle may return with gold, treasure or fish(if near water).
Turtles will refuse to be rideable if there is a thunderstorm, and rush into a hut(One hut for 1 turtle)
If they cannot find a hut, they will just lay down until the storm is over.
In water, they are faster than rafts, but slower than a Karve. They have less health than trolls.
Turtles frost breath is the same as Moders but deal way less damage(Same as a draugr arrow?)
Turtles in wandering mode will attack Black forest mobs, meadows mobs and swamp mobs.
While riding a turtle, LMB/R2(changeable in settings) does the frost breath attack(5s cooldown) and RMB/L2(again, changeable in settings) does a stomp attack, similar to Lox.
Just had this idea all of a sudden. I luv turtles :3.
TL;DR Cool ice breathing, rideable dragon turtle go brrrrr
Source: https://www.reddit.com/r/valheim/comments/qf6fnt/rideable_tameable_sea_turtle/