Ms Hirst was riding on Hay Bluff when she saw the lamb feeding from the pony.
The equine vet from Hay on Wye said: 'I was out riding one day and I spotted a baby lamb feeding.
'The first time I saw it I couldn't quite believe it - I thought I might be imagining it.
'But then I saw it again and again over the space of about a week.
'It seemed the Shetland mare had adopted the orphaned lamb and was letting it feed from her.'
Ms Hirst said Shetlands are not normally amenable creatures and often struggle to rear foals.