Upside-down letters, as in ∀, and left-side-right letters, as in ∃, are used in mathematics and logic to indicate for all and there exists. Do you know other examples of such inversions in other fields. Is there a name for the practice?
There are many other examples in mathematics alone, and the answer will depend on what transformation you allow and what font you choose. For example do / and \ count as transformed I's? The schwa (ə) for phonetics comes to mind. You might want to check out the full IPA symbol list and the unicode block of letter-like symbols, U+2100 through U+214F, which contains several turned letters. Various languages use transformed Roman characters. For example Cherokee has a backwards J.