Apple Air tag for dogs:

Apple did not design AirTags for tracking ... either people or pets ... so, finding an AirTag holder for a dog collar would not be something that Apple will sell either at their online or physical stores.

Also, be aware that an AirTag will start beeping when it is away from your iPhone after a certain amount of time. Whether or not, this beeping will affect your dog is something that you should take into consideration. For example, if you have been using a "training" collar with your dog, the AirTag beeping may confuse them.

Do understand the limits of the technology — that all airtag position finding is determined by the position of a “relaying” Apple device.

So … if your dog gets loose and is fairly near an iPhone, you might be able to locate him (or at least see where he WAS) …

… but if he runs off into the woods or is loose w/o relaying devices very nearby, an Airtag isn’t going to help much.

fit for a dog is a broad term I use this one

AirTag Forest Green Leather Key Ring - Apple