Scientific research is continually studying the possibility of zombies. It is generally accepted by the scientific comunity that zombies as seen in the trids do not exist.
There are a few instances which beg the question though.
Cyberzombies: Through mixing dark magic and technology, one can ignore the natural limit of body modification. Given the proper emotional disorders, a cyberzombie could theoretically play the role of your standard horror trid monster. There are numerous safeguards which prevent this though.
Possession Magic: The proper type of spirit can possess a deceased body. Depending on the spirit it could play the role of a zombie as well.
VITAS Infected: When various metatypes become infected by VITAS they develop a dietary requirement for metahuman flesh. They need fresh flesh to survive, but they are not the standard slow and shambling creatures suggested by the trids. They are extremely tough, and many of them retain their minds.