While Ogres and Hill Giants suffer an AC drop from AD&D to 5E, I see no reason we should apply the same to this poor "Xyclops".
I'd prefer its AC to be more "absolutely scaled" based on the original AC rather than matching a 5E creature that has a significantly worse AC than it did in AD&D.
Heck, the 5E version of the Fomorian doesn't even wear armour, while the AD&D version wears so many scraps of armour it has AC 3, the equivalent of plate mail!
So I'd prefer an AC of 13 or 14 rather than 12. I'd go for 13 (natural) myself, as (a) that's the same as an Orc in both AD&D and 5E, (b) it give the Cyclops an AC two lower than a Troll in both AD&D and 5E, and (c) it's a point lower than the baseline AC 14 of a Challenge 4 monster according to Monster Statistics by Challenge Rating in 5E's
Dungeon Master's Guide (page 274, or see this online
CR Calculator).
Note that the AC differences can go both ways. While 5E Ogres and Hill Giants are a lot easier to hit than they were in AD&D, 5E Goblins and Bugbears are a lot harder to hit.
For comparison, consider the Armour Classes of some standard-issue giants and humanoids:
Monster | 1E AD&D¹ | 2E AD&D | Fifth Edition |
---|
Goblin | AC 6 | AC 6 (10 unarmored) | AC 15 (leather armor, shield) |
Orc | AC 6 | AC 6 (10 unarmored) | AC 13 (hide armor) |
Bugbear | AC 5 | AC 6 (10 unarmored) | AC 16 (hide armor, shield) |
Ogre | AC 5 | AC 5 (natural armor²) | AC 11 (hide armor) |
Troll | AC 4 | AC 4 (?) | AC 15 (natural) |
Hill Giant | AC 4 | AC 3 (5 natural armor) | AC 13 (natural) |
Fomorian | AC 3 (1 or 2)³ | AC 3 (1 or 2)³ | AC 14 (natural) |
Stone Giant | AC 0 | AC 0 (natural armor) | AC 17 (natural) |
Frost Giant | AC 4 | AC 0 (5 natural armor) | AC 15 (patchwork armor) |
Fire Giant | AC 3 | AC –1 (5 natural armor) | AC 18 (plate) |
Cloud Giant | AC 2 | AC 0 (natural armor) | AC 14 (natural) |
Storm Giant | AC 1 | AC –6 (0 natural armor) | AC 16 (scale mail) |
1. In 1E AD&D, the
Monster Manual (1977) does not say what these monsters ACs are without armour.
2. The 2E AD&D
Player's Option: Skills and Powers and the
Complete Book of Humanoids both say an Ogre's natural armour class is 5, despite an ogre's
Monstrous Manual entry implying they wear armor ("Dressing in poorly cured furs and animal hides, they care for their weapons and armor only reasonably well"). Presumably if their natural AC is 5 a suit of leather or hide armor does not improve their AC, but is just warmer.
3. An AD&D Fomorian wears scrap armour which normally gives AC 3, but if it has better scraps or a shield if may have AC 1 or 2.