Does anyone know of any pathfinder books that details the primal inevitables’ physical appearances, specifically Kerkamoth? Kerkamoth deity page
Kerkamoth was introduced by Concordance of Rivals, one of the later books. Since Pathfinder Wiki lists only this book as source, I guess it's the only PF1 book covering them.
Gods & Magic of Pathfinder Second Edition has them too - no idea whether this book shows a picture. The PF2 forums here should be able to answer that.
Finally, if you ask as a GM, your players are likely content with any image that more or less fits to Kerkamoth theme.