Knight on horseback
Humanoid figure with lance and shield on top of a horse.
This fun model is very recognizable and has many options for modifications. For example, the weapon and shield can be changed to wings, making the horse a pegasus; the horse can be made a unicorn; or the lance (or shield) can be changed into another weapon.
Paper & size
This model ends up 10cm long and 8cm in height when folded from a 26cm square.