Create a Creature
Tinkercad is the easiest way to design your own 3D stuff!
Thingiverse is a big library of 3D models that you can download and print.
Other sites like thingiverse:
Strategy #1 - "From Scratch"
Use Tinkercad to create a creature from scratch. This method is fun and fairly easy, and your creature will come out playfully cute.
Use basic shapes, like spheres and cylinders, to assemble a simple creature.
Add your adaptation. Make sure it is obvious and noticable on your model!
Strategy #2 - "Import and Modify"
Use thingiverse.com or a similar online model collection to find a base model of a creature to modify. This may seem like the easier method, but finding a model that will print well can be difficult. Thingiverse is a very large website that is often slow or difficult to use.
Find a model that looks relatively simple and has large features. Looking for models that are called "low poly" tends to help, as these models have less details (larger triangles) making up the shape.
download the .stl file from the "Thing Files" tab
Import the .stl into Tinkercad and make your adaptations!