The Wittle Widowling is a tier 3 trinket found from harvesting the Black Widow Web Sacs in the Black Widows den in the Brawny Boy Bin. It behaves similarly to Mom Genes, with attacks and perfect blocks summoning friendly Black Widowlings that attack foes and eventually die after living for a period of time. The summoned Black Widowlings are also capable of inflicting Venom.
This trinket can be extremely hard to see when it drops from the web sac (you may need to point your cursor under the floor where the web sac was). If you did not pick it up when you first killed the web sac, odds are good that the trinket never despawned and is still lying almost invisible in its spawn location.