As stated, the Sherean Leviathan is the giant sea monster at the back; unfortunately, it is only in the set for those who pre-ordered it, so there is a chance that you weren't able to get it based on when you got the set. Metatron is of course the angel with the tall wings.
As for the other creatures, I don't think that they were ever planned in the set. Keep in mind, the ForceMaster vs Warlord box cover has creatures that weren't included either. Heck, even the Warlock's helmet shown in the core set that he is wearing wasn't released till it was put in the Forged in Fire set. All in all, the multiple creatures on the box art are usually just there for show and to grab your attention.