The unicorns have begun to starve, but the 3-Z tall wall with a perimeter overhang seems to be keeping them safe from the goblins. I read that some animal populations have a number in the raws that means if X number of those animals are killed, you won't get any more visiting your site ever... With a new access tunnel dug into their pen, it seems that their trainers are bringing them sufficient food to survive for now. None of them have a "hungry" indicator in their wound status screen, so crisis averted? It was DFHack that notified me of their starvation.
Since I embarked in a good biome, I didn't expect much trouble, and thus ignored preparing the usual air tight defenses. But 40 goblins with 20+ beak dogs arrived after the 2 werebeasts killed all my fresh recruits. I sealed the direct access routes off, but the goblins and beak dogs have been randomly jumping the 1 tile wide, 2-Z deep dry moat and getting into the fort. I didn't notice until they killed 20 before another batch of recruits were drafted to stop them. Some doors have been put up to keep the rest out (no trolls in this invasion) while the new recruits train, and cage traps built between burying the dead to help even the odds a little.
The 7 unicorns have been assigned to golden chains (matching their golden eyes) in their pen to hopefully prevent future starvation. The wiki recommends 38 tiles per unicorn, but 8 tiles accessible from the chain is better than 0 in a cage.