If you didn't know this...these dinosaur planters have been a hit on Etsy. At least from what I have seen. They are very easy make and look really rad!
First...you will need a plastic dinosaur. You can find these at Walmart for a dollar and larger ones at Toys R Us. I painted my dinosaur with acrylics and sprayed with a finish. I wanted a complimentary color palette for these photos.
Take your Exacto Knife and cut a hole around the top (well...back) of the dinosaur. It is just like carving the top of the pumpkin...except this is plastic.
The hole I created is around 3-4 inches. Thankfully the dinosaurs are already hollow inside.
Then, I put in my plant and gave it water so I could expel some of the water using my sewing needle. I made plenty of holes in the bottom of the dinosaur...you have to apply a lot of force for this part.
Your project should look similar to this in the end after you have added soil and your plant. :D