There are a large variety of animal tags, including some with similar configurations of those that are implanted in humans.
TL;DR - materials, manufacturing, and Q/A checks
A “human RFID implant” has only existed once before… VeriChip got FDA approval in 2004 for their product. We started Dangerous Things in 2013 by sourcing and testing “animal” implants from various factories (mostly in China, but not all) and the results were … less than encouraging. Toxic epoxies… impurities in glass… it was bad. Then we crowdfunded the xNT, got our own materials together, got connected with a factory we could implement clean room requirements with, and implemented additional QA checks along the manufacturing process. Our stuff is not FDA approved, but we’ve done more testing on our implants than even most “animal” chip suppliers out there.
Don’t even get me started on industrial grade glass tags… the stuff you find in car keys and what-not are terrible and should never be implanted.