Copper

Copper is originally Lou's horse.

When Mallory develops a connection with him she and Lou agree she will work for sweat equity to become partners in Copper.

Season 1
Mallory overhears Lou talking to a client about selling a horse, she believes to be Copper. She ends up taking Copper so that he can't be sold but instead stumbles upon a feed lot full of mustangs. (Born to Run) Mallory is brushing Copper in the barn with the electric heater when Jack tells her off for it being dangerous and says it's not cold enough to justify the cost. When Wes starts the fire in the barn Jack manages to get all the horses out unharmed before he gets injured himself. (Rising from Ashes)

Season 2
Mallory tries to train Copper to jump and runs ground poles but is disappointed when he keeps kicking them. (Corporate Cowgirls)

Season 3
When Ty goes missing with Scott, Mallory uses Copper as a source of comfort. (Eye of the Wolf) Mallory rides Copper when out with Amy and Neon. She later rides him to help round up Tim's cows from the Dude Ranch and get them back to Big River. (Catch and Release) Mallory rides Copper to Marion's memorial. (The Reckoning) Copper gets strangles after it spreads around Heartland, Mallory helps look after him. (Quarantine)