Cows.

A friend of mine was taking over his family farm with access to open moorland for grazing and as an experiment we wanted to see if we could implement tracking without giving a monthly small fortune to a company for a proprietary product or noisy polluting methods like a helicopter. BLE beacons are cheap and long-lasting so each cow got one, and we bodged together about twenty solar powered Meshtastic nodes with GPS that were small enough to be worn on their collars (since the cows tend to heard together only one in the group needs a node), plus another dozen we could install at choke points on the land (old gates, land bridges, etc) the cows were likely to traverse. The nodes were like this, but smaller and shitter. The nodes were programmed to report every time they saw a BLE beacon nearby, with the mobile ones also reporting their own GPS location, all being fed into Home Assistant for mapping. I just checked and apparently, aside from a couple of storm-damaged fixed nodes and replacement batteries it’s still going strong after two years.