The animal was trapped in its carriage in Park Road, Gosport.
Firefighters received calls saying the animal was stuck after 12.30pm.
The partition in the box fell, trapping the animal inside.
Specialists animal rescue teams from Lyndhurst station attended.
They went into the horse box and removed the partition.
A vet then mildly sedated the horse and it was led to safety unharmed.
Officers from Gosport fire station were sent to the scene and were soon followed by firefighters from Basingstoke and Brockenhurst.
Hampshire Fire and Rescue’s animal specialist unit has also been sent to the scene.
A spokesperson said: ‘We were called to Park Road after it was reported a horse was stuck in a trailer.
‘Our animal rescue specialists are there.’