Who was Zhang Sanfeng?

Zhang Sanfeng was a semi-mythological Daoist figure who is credited with all sorts of things, from acupuncture to Taijiquan.

The phrase "semi-mythological" refers to the possibility that he may have been based on a real person. Actually, people who have done better research than I have suggest he may be based on two different people. Regardless, he has been credited with a lot of stuff he didn't do.

Did Zhang Sanfeng invent Taijiquan?

No. Zhang Sanfeng is credited with inventing the 13 Postures, of which Taijiquan uses a specific variation. I'll publish a post about the "Old" 13 Postures soon.