Barghests gain strength as they eat others. "For every three suitable corpses a barghest devours, it gains 1 Hit Die, and its Strength, Constitution, and natural armor increase by +1". (source)
(Once they reach 27 such corpses, they evolve into a greater Barghest, weirdly losing strength in the process).
To be clear, like I said earlier, barghest have multiple issues - no teleportation, no real explanation for the circus scene, and even at 26 corpses, it's strength is 25, which is nothing to write home about.
ETA: I am unsure what version Eric the White is talking about, but the ones in the FR wiki don't seems to come with stats, so not sure how they differ from the d20 srd one which is my usual go-to for official versions of monsters.
GW