So the PHB and DMG don't do a good job of clearly explaining how subordinate creatures work. It seems like it should all be one subject, but instead it's spread out in a dozen different places and only discussed as a side-note, if at all. I feel almost like it either wasn't given much consideration by the developers, or was actively being avoided/suppressed as a non-standard style of play.
There's dogs, horses, and other "mounts" for purchase in the PHB, and there are rules for how to use them as mounts, but no explanation of how they work otherwise.
Aside from purchasing animals, there are a few other ways to acquire subordinate animals, including the beastmaster's animal companions, the urchin's pet mouse, steeds from the find steed spell, or familiars from the find a familiar spell. These each have their own rules, but there is no explanation for how- or if- these creatures operate in the same way as some animal you bought in addition to those rules. The implication is that they are each a separate system entirely, despite being able to coexist, and possibly overlap the same monster entry in some cases.
The DMG lists the food and water needs of creatures by size. I can't find a chart incorporating care expenses into life expenses. The purpose seems to be intended for surviving in the wilderness, but the needs of subordinate creatures are not considered by the rules at all otherwise, nor is it stated whether creatures born from find steed or find familiar need to eat.
The PHB also lists the prices for hirelings, and the DMG lists uses for hiring them to maintain various types of property. There is absolutely no discussion about them outside of those two lists- not even an explanation of the term "hireling", which I'm assuming has the same meaning as back in basic D&D where they were a core part of the game.
Nothing in the book states whether these things are NPCs, PCs, or something else entirely, and no advice is given on how they should be played.
Am I missing something that would explain the apparent inconsistencies, or act as a sort of root-rule that is being modified by all of these separate sections?
This question is similar, but is purely from a player's perspective and for roleplaying purposes only: Are there any documented rules about owning vanity pets in D&D?