How do I make animals or mobs behave differently in the game?

Use Tynker's mobs editor! Of course, you can change how an animal looks… But also how the animal behaves, using code blocks. Choose a mob to start.

https://www.tynker.com/minecraft/mobs/

Click on the code icon in the upper right corner to edit mob behaviors. If the code icon isn't shown, editing mob behaviors may not be available for that specific mob. Try a different mob instead.

Hint: The starting Mob you choose matters, especially if you're changing the 3D model. Choose a similar shape and body type to what you have in mind, if one is available. This will ensure that the new mob animates well within the Minecraft world. (If you make major changes to the structure of the starting model, you will likely see unexpected mob animations.)