minibuffer-history-case-insensitive-variables

minibuffer-history-case-insensitive-variables is a variable defined in `simple.el'.
Its value is

nil

Documentation:
Minibuffer history variables for which matching should ignore case.
If a history variable is a member of this list, then the
M-x previous-matching-history-element and M-x next-matching-history-element commands ignore case when searching it, regardless of `case-fold-search'.

You can customize this variable.