Trouble making a crunchy dog treat
Right, I'm not a hands-on baking expert, but some of the reasons your gluten may be having trouble are that

the gluten-containing flour makes up a quite small proportion of the overall mixture,

you have other ingredients (namely: the oil, the fat in the peanut butter, and the wheat bran) that act to minimise your gluten development and

there's very little moisture in your mixture


Not much flour
This one's pretty straightforward; the less of your dry mass of ingredients is wheat flour, the less glutenin and gliadin (the precursors to gluten) you have in the mix, so the less gluten can be formed. It's possible that the amount of flour you have now is sufficient though, if its gluten development is not further sabotaged by the other factors. If you want, you might replace some of the ground rice, wheat bran, and ground peanuts with more regular flour, though I'd probably try that after I tried some of my other suggestions.
Anti-gluten ingredients
Fat has the effect of both lubricating the components of gluten against one another, making them more likely to slip and not form the tough, elastic mesh we know and love from our breads, and also of isolating the gluten from water, which it needs to form properly; there's a reason gluten doesn't form in a bag of flour sitting in your pantry and that reason is there's no water to activate it. The fat can coat your glutenous proteins and make it harder for the water to get in there and do its thing. Wheat bran, separately, has a bit of fat in it, but also has a lot of 'sharp edges' on a small scale, which can slice through gluten strands as they form and inhibit, again, the protein mesh from forming. Cutting back on these ingredients might help the gluten develop somewhat, though I'd still try that after my third suggestion, namely:
Add some water
As I said, gluten needs water to develop; bread doughs regularly have flour-to-water mass ratios of 2:1 or even 3:2, which would correspond to baker's hydration percentages of 50% or roughly 70% respectively. I don't think this recipe needs that much water, but right now the only ingredient in your list that I can see is adding any  free water at all is the peanut butter, and even that isn't much. I imagine your dough for your past experiments has been very 'tender' and crumbly, barely held together by the fat content, more than the water and gluten. Try adding water, maybe a half cup at first, and probably more, until you have something you can vaguely knead. It won't be anywhere close to as stretchy as normal bread dough, due to all the non-flour ingredients, but there should be some amount of stretch to it.