It happened just before 20:00 BST, Central Motorway Police Group (CMPG) said.
Officers shut the motorway in both directions between Junction 4A and Junction 5. Three horses and a foal were found on the northbound carriageway.
CMPG said all four horses had now been returned to a field.
It tweeted: "Full closure both ways M6 at J5 due to 3 horses in the road. First time I've reported that in Birmingham!"
The M6 was closed outside Birmingham after horses were spotted on the motorway.