Species-ID: What is this thing attached to my wall (California, USA)
This is very likely a chrysalis (or pupa) stage of a lepidopteran.

A similar looking example belongs to the giant swallowtail butterfly (Papilio cresphontes).



 Source: Bill Frank 

P. cresphontes is found in southern California [source], so it would not be unreasonable to find it in Simi Valley.