Reach Should Always Be Listed
Operative word here being "should" - I've not yet seen a creature where it seems like it was a mistake it wasn't.
The rules for reach as it corresponds to size say this:
Table 9–1 also lists the typical reach for creatures of each size...
Table 9-1: Size and Reach
Size |
Space |
Reach (Tall) |
Reach (Long) |
Tiny |
Less than 5 feet |
0 feet |
0 feet |
Small |
5 feet |
5 feet |
5 feet |
Medium |
5 feet |
5 feet |
5 feet |
Large |
10 feet |
10 feet |
5 feet |
Huge |
15 feet |
15 feet |
10 feet |
Gargantuan |
20 feet or more |
20 feet |
15 feet |
The table shows the typical reach of a Huge (Long) creature like the Greater Nightmare is 10 feet. However... note the word typical means that this is more of an FYI of what to expect than an actual rule. These rules in turn reference the Range and Reach rules on page 455 of the CRB:
Your reach is typically 5 feet, but weapons with the reach trait can extend this. Larger creatures can have greater reach; for instance, an ogre has a 10-foot reach.
Again, none of the language here implies that it's all Large or larger creatures have reach! Importantly, the referenced Ogre explicitly calls out Reach in its stat block. This would be totally unnecessary if we were to assume the typical Reach.
RAW, Reach is never assumed to be anything but 5 feet, and thus must be explicitly printed on a stat block. Thus you should assume no printed Reach means 5 feet, even for Huge or larger creatures.
This is further reinforced by the text of the Enlarge spell:
Bolstered by magical power, the target grows to size Large. Its equipment grows with it but returns to natural size if removed. The creature is clumsy 1. Its reach increases by 5 feet (or by 10 feet if it started out Tiny), and
A PC with this spell is presumably now a Large (Tall) creature. Should the above table apply to all creatures regardless, then you'd already have a 10 foot reach just for being Large! This would mean Enlarge gives you a 15 foot reach - which I suppose isn't out of the question, but it very clearly seems to be designed to keep PCs on pace with the typical reach of Tall creatures of various sizes. Again, if the typical reach from the table was to be assumed, this text would be unnecessary.
It seems reasonable to say that, RAI, it's left up to the GM to determine whether it makes more sense for the creature that doesn't mention Reach to have the default Reach or the typical Reach - there's a lot that goes into designing a creature, and it's easy for authors to miss small details, especially if they were assumed in other editions.