Insular S

The letter Ꞅ (minuscule: ꞅ) is an Insular script form of the letter S. The uppercase is encoded in Unicode at U+A784 [1], and the lowercase is encoded at U+A785 [2].

Insular "s"

Evolution of the Insular s from sigma: ς-s-ʃ-ſ-ꞅ

It should not be confused with the long-legged r, U+027C ɼ LATIN SMALL LETTER R WITH LONG LEG, which is an obsolete IPA symbol.