Numbers hang off the necks of those left behind.
They're spray painted on those found starving at the 3-Strikes Ranch, aka Mustang Outpost. (Thankfully, Jason Meduna, the man who starved so many mustangs to death, was convicted of animal cruelty. Too bad his sentence didn't reflect the suffering he inflicted.)
Numbers are freezebranded onto hips of those deemed 'too old' to adopt.
Even the wee ones are numbered and tagged.