It could be that the puppy has some physical issue or is imitating another animal's walk for fun. Sometimes, cartoons do things that are a bit out of the ordinary for entertainment purposes.
It might be a unique artistic choice to create a distinct look or to emphasize certain features.
Maybe it's because the creator wanted to show that the dog is very energetic and agile.
It could be a creative choice by the animators to give a unique twist to the classic image of the little mermaid.
The reason could be to give Grimlock a softer and more approachable appearance. It might be a creative choice by the artists to add a touch of charm and evoke certain emotions from viewers.
One of the common cartoon animals that walk on two legs is the famous Peppa Pig. Also, many superhero animals like Spiderman's pig counterpart might be shown walking on two legs. It's all for the fun and imagination in the cartoon world.
Maybe it's part of the character's design to make it stand out or add some mystery.
Maybe it's for a unique style to make her stand out.
The reason could be to catch the viewer's attention. Crazy eyes in a cartoon face make it more unique and likely to draw people in. It might also be part of a story or theme where such expressions are needed to convey a specific message or mood.
Maybe it's for a unique and creepy look to make the character stand out.
Maybe it got injured or has a genetic issue. That could cause problems with its hind legs.