Are all emerging viral diseases of the past 100 years zoonoses?
To my knowledge, yes. A partial list of recently emerged/emerging viral diseases (I certainly could have missed some), with probable reservoir hosts:


Chikungunya* (birds, rodents)
coronaviruses (SARS [bats], MERS [camels], COVID-19 [?? bats ?? pangolins ??])
Ebola and other filoviruses (Marburg): (bats?)
Hendra, Nipah (bats)
Ross river virus* (various mammals)
HIV (primates)
influenza (H1N1, avian) (birds/pigs)
monkeypox (monkeys, duh; also rodents)
West Nile virus* (birds)
Zika* (? "a wide range of animals in West Africa")


Starred examples are vector-borne (so perhaps of slightly lower concern - might not fit your criterion of "capable of causing a global pandemic").

Omitted: 


older zoonotic viruses (rabies, dengue, hepatitis, ...)
non-viral zoonoses (malaria, plague, anthrax)


A list of zoonoses; another from US CDC

More generally, the only other place an emerging virus could come from would be from mutation or recombination of existing human viruses. I'm not aware of such an example.