An old mare who has never raised a foal has become an unexpected foster mum, after she adopted an orphaned calf in outback Queensland.

Cattle grazier Gerda Glasson of Piccarilli Station, near Thargomindah in the state's south-west, said she was checking horses in the paddock when she discovered the less-than-a-week-old calf with Moonshine the horse.

"I'm thinking the calf must've got lost and I was looking around for the mother cow, but [the] calf got up and started walking over to Moonshine," Ms Glasson said.

"Moonshine put her nose down as if to say, 'this is mine', and the calf started trying to drink.

"It was something I've never seen before."

Moonshine is roughly 16 years old and has only ever foaled once, a decade ago, but the foal died.

"She's never actually raised a foal; she's never been a mum," Ms Glasson said.

"So she might be thinking that this is her chance."

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The Glasson family recently moved some cattle and while they reconnected offspring with their mothers, they suspect the calf became separated during that time.

The orphaned calf was still in good condition despite not having had a drink in a little while, Ms Glasson said.

Moonshine is not producing any milk and for the meantime, the graziers are bottle-feeding the calf in their backyard, although its adoptive mother is keeping a close eye on events.

Ms Glasson said she had never seen anything like this situation except for stories in which orphaned animals were deliberately placed with mothers who had lost their offspring.

"Moonshine has just picked this one up on her own but she's going to have to give it up very soon," she said.

"She's our kids' polocrosse horse and that's starting up so she's going to have so say goodbye very soon.

"I can't see myself dragging a calf around to the polocrosse carnivals — it'll be a bit of a sight."

The calf is yet to be named, but Sunshine, Sunny, Horse and Pony have all been suggested by Ms Glasson's children.