Zoan type devil fruits give users the abilities of an animal. They can transform into a full animal form and a halfway between form as well as normal human form.

Chopper here was a reindeer who ate the devil fruit that gave him the abilities of a human. After studying his abilities for a while, he was able to create a pill that allowed him additional transformations hi-lighting different abilities.

The little one in the center is his in-between human and animal form, which he spends most of his time in.
In the bottom left is walking point (a.k.a. reindeer form).
The bottom right is heavy point (a.k.a. "human" form).
The middle left is jumping point.
The middle right is horn point.
The top left is arm point.
The top right is guard point.
The shadowy figure in the back is the result of an overdose and is often referred to as "monster point" but I'm not sure if that canon or fanon, to be honest.

This is actually made to be a poster, it's poster-size at 18x24, 300 dpi and CMYK color, but I'm not sure I'll print it. I worry that the crunchy coloring might not look good big, I kinda like it, but how will it look blown up?

I don't want to admit how long this took me to do, but as an indication, I started it before I started the bookmarks: [link] and that's also why Chopper didn't get a bookmark, because he was getting this poster.

Mostly, I just wanted an excuse to draw his various transformations, because I love the designs. (Except brain point, it looks just like his normal form, so there's no point to drawing it, heh.)
I have to give many thanks to Unlimited Adventure and it's gallery feature, though. The game had 3D models of ALL of his forms, and you could move around them to any angle and zoom in and out, it made it so easy to draw these. God, I love that game.