I'll answer both replies with one thing, even though one of them is mine.
Croagunk has dry skin and Anticipation, and dry skin is related to eeczema(i think) and Anticipation starts with A, AND it's pokedex entries for D/P/Pt/B/W all start with I, so that coveres sczema and entei.
and, the letter is k, cause it solves the separation problems.