The SRD says: "The GM makes one roll for an entire group of identical creatures, so each member of the group acts at the same time."
Note: the casting time for Animate Dead is 1 minute, so it's unlikely to occur during combat. But if it could, you might use the precedence from the many Conjure spells (like Conjure Animals), which say "Roll initiative for the summoned creatures as a group, which has its own turns".
Personal experience: despite the above, every group I've played with found it intuitive/easy to just handle these things on the caster's turn as a home rule.