Most plants, certain animals and certain meat or meat products are not allowed in under circumstances. Some meat products are allowed in when in quantities for personal consumption.

I regularly bring in candy, chocolates, boxed cereals, etc., but always declare them. I've also brought in baked Chinese 'bao' or buns filled with pork, but I think the inspector thought they were pastries and not meat, but I'm not sure.

I believe you can check the US Dept of Agriculture website for a more comprehensive listing of what is forbidden.