[RPG] Resource with Monsters Listed by Type

In this case I would consider the term "Favored Enemy" (from the phrase "one additional favored enemy") as referring to the class feature itself, with the text:

Choose a type of favored enemy: aberrations, beasts, celestials, constructs, dragons, elementals, fey, fiends, giants, monstrosities, oozes, plants, or undead. Alternatively, you can select two races of humanoid (such as gnolls and orcs) as favored enemies.

So when you gain that class feature again you would gain exactly what it says, every time you gain it; unless the feature says otherwise, of course (e.g. you may not choose the same favored enemy twice).

Using an online Challenge calculator, I am not seeing the issue that you are describing.

Monsters exist up to CR 24, and then the Tarrasque by itself is 30.

For 4 Level 20 PCs, a single enemy of CR 23 is at the high end of a Hard encounter, just barely below the threshold for a Deadly encounter. A CR 24 is thus a Deadly encounter, but 5 Level 20 PCs push it back down to a Hard. (The Tarrasque requires 7 Level 20 PCs to push it under the Deadly level, but lets ignore that one! Using that, you are definitely threatening a TPK!)

I believe you are calculating challenge improperly. I will explain what I think you are seeing, and you can obviously correct me if I am off base.

For the CR calculations(using the table on page 82 of the DMG or page 56 of the Basic DM Rules), a level 20 PC has a Deadly threshold of 12700 XP. A single CR 15 monster is worth 13000 XP. Since that is higher, you see it as being a Deadly encounter. However, for encounter calculations, you must add in the XP threshold for each party member to arrive at your overall thresholds (which is explained in step 2 of the Evaluating Encounter Difficulty section on the same page as the XP Chart). For a party of 4 Level 20 PCs, the Deadly level is set at 50800 XP. A single CR 15 monster against a party of 4 Level 20 PCs will be an Easy encounter.