The oldest known form of Taijiquan evolved from Shaolin Kung Fu and was developed by Zhang San Feng (13th century) at the Wudang Mountain.
Unknown outside of China and presently bears little resemblance to all the Chen family linked styles.
The forms are graceful and agile, with force flowing continuously without a break, like flowing water. No mechanical strength required for this style.
There is a modern style by the name Wudan, but this is a more practical, self-defense oriented style similar to Wu Style Taijiquan.