Preheat oven to 350 degrees. Line a baking sheet with parchment paper.
Shread the zucchini, carrot, and spinach in a food processor or with a hand grater, or use a knife to cut the veggies into a fine mix. Make sure the veggies are in a fine mix, I cut them on cutting board to get that fine mix so you get them well mixed into the dough.
In a bowl using a hand or stand mixer using wire attachment, beat the canned pumpkin, peanut butter, and eggs on medium-high until mixed well, about 1-2 minutes.
Next, add old-fashioned oats a little at a time and 2 ½ cups flour at low speed, mixing in well. Add an additional ½ cup flour at a time just unit the dough is no longer sticks to the bowl. Add the shredded carrot, zucchini, and spinach, until well distributed in the dough mixture.
Flour your work surface VERY WELL add way more flour than you think... and knead the dough 3-4 times until it comes together. The dough is tough and sticky! That is ok... use a rolling pin to roll out the dough to ¼-inch thickness. Keep adding flour so the cutter won't stick, and the Doug won't stick to the surface. Using a personalized bone shape cookie cutter, or any shaped cookie cutter you like, cut out and place onto the prepared baking sheet.
Bake until the edges are golden brown about 30 minutes.
Then turn off the oven heat, and let the biscuits cool and dry in the oven overnight with the light on in the oven.
Dry the dog treats dry well for a few days on the kitchen counter, then store the dog biscuits in an airtight container in the freezer for several months. Simply pull from freezer, set on counter for 20 minutes before serving. Your dog will LOVE them!