It would be great if there was a skill called animal training.
With animal training you can train animals such as dragons or sea serpents and if you train them successfully then you can buy a whistle and when ever you blow the whistle that animal will come to you.
There could be some other skills too such as:
1. Animal riding (lets you ride trained animals)
2. Animal care (lets you care for your animals better)
etc.
Here is a simple Idea of how the dragon will look like:
i.imgur.com/23GTy.png
Sorry but the sea serpent is still in progress but I will post it when I'm finished.