I've heard a lot of people talking about the pounce evolution being overpowered for unfettered eidolons (Bestiary 3), and while it is very strong for normal summoners, I don't see it in the list unfettered eidolons can take. Is there something that let them take normal summoner evolutions, an update I've missed, or something else that lets them take pounce?
We need to clearly delineate between PC's "eidolon" class feature and monster "unfettered eidolon".
Two Different Eidolons
If a player is a summoner, the rules for building their eidolon are outlined in the Eidolon class feature's description in the APG and supplements. The Unchained Summoner class from Pathfinder Unchained has a similar but different eidolon ability as well.
Bestiary 3 also introduces a monster called the Unfettered Eidolon. The rules for creating a unique unfettered eidolon are given in the monster's description.
Observe that these are two completely discrete sets of rules: neither references each other.
RAW: No Pounce
The rules for creating an unfettered eidolon are completely captured in the monster's bestiary entry. Pounce is not on the list of available evolutions, so it is not a valid choice.
This is slightly confusing because the very similar eidolon class feature includes pounce. However, these are different sets of rules that do not interact.
A more familiar example might be the Druid's animal companion: you could opt to have a boar companion, and there is also a boar in the Bestiary, but these two things are entirely different.
However, it would be easy to think that pounce was accidentally omitted from the list of evolutions for unfettered eidolons. The GM could include it in the list.
This wouldn't benefit your PCs at all, but would allow the GM to give pounce to encountered monsters.