Petyr Baratheon

Petyr Baratheon is one of the stillborn sons of Stannis Baratheon by his wife Selyse Florent. His mother keeps his corpse in a glass urn in her room along with those of his equally stillborn brothers, Tommard and Edric.

In the books
In the A Song of Ice and Fire books, it is only mentioned that Stannis and Selyse have no children other than Shireen. There's no mention of any stillborn sons - this information was introduced by the TV series. Staff writer Bryan Cogman came up with the idea of the stillborn fetuses being preserved in glass urns filled with chemicals.