A surrogate mother is a woman who carries and gives birth to a baby for another person or couple. She carries the pregnancy on behalf of someone else, usually because they are unable to have a baby themselves. Full definition
Sheep are sometimes used as surrogate mothers for orphaned rhinos; Gertjie and Matimba eventually formed a herd of three with Lammie the ewe. (discovermagazine.com)
The same consideration that SPS gives to surrogate mothers in choosing their intended parents is of course given to the intended parents when searching for a surrogate. (surrogateparenting.com)
The lactating mares are then used as surrogate mothers for foals from other horses. (kyhumane.org)