Behemoth

Optional and soft-missable. Behemoth turns up in the Great Desert arena via the "Encounter" side quest, which itself needs Kaya's questline going (Affinity 2 - see Sisterly Love). So it's gated behind the same Xion window: gone once you advance past Xion after Abyss Levoire, recoverable only in NG+. A big brawler - dodge the unblockable slams and punish in the gaps.