Prince was Perdita's mate in the novels.
The Original Novel
Prince was a valuable liver-spotted dalmation. He was at a family picnic when, as his owner was offering him a chicken, he saw Perdita. Abandoning his chicken, he and Perdita raced into the woods where they married. Prince insisted that she live with him, but upon returning their owners tore them apart. Perdita later had his puppies.
At the end of the book, Prince was reunited with Perdita and sold to the Dearly's, becoming the hundred and first Dalmatian.
In the Musical, Based on the Novel, Prince acts as a narrator introducing the Audience to the characters. Perdita later sings about him during her song; "One True Love".