If a doppelgänger or changeling meet a werewolf in their hybrid form, could the doppelgänger/changeling then mimic that form?
Technically, a werewolf is a humanoid. But Shapechanger is an ability.
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The Doppelganger's Shapechanger trait states:
[...] Its statistics, other than its size, are the same in each form. [...]
And the Changeling's Shapechanger trait states:
[...] none of your game statistics change. [...]
We know "What all is included in a creature's game statistics?" which is basically everything about an NPC, monster, PC, or anything else really. And these features cannot change your statistics. So while you might be able to look like a hybrid form that you have seen, you would not gain any benefits that that creature usually has.
Doppelgangers can turn into into any Small or Medium humanoid they have seen and Changelings can turn into any creature they have seen, if it has the same basic arrangement of limbs - which includes the werewolf hybrid form, since it is a creature with the same basic arrangement of limbs.
That said, their stats stay the same as in the normal form: this feature is designed for infiltration, you mimic neither their existing equipment, nor their traits and statblock.