The new Livestock Identification and Commerce Act took effect January 1, 2009.

Section 19(2) is summarized for us by Alberta Agriculture and Rural Development:

A livestock manifest is not required to transport horses within Alberta unless they are being transported for sale or slaughter or to an inspection site (feedlot, abattoir, auction market or country sale).

But please take the time and read the full text.

Source:
Livestock Identification and Commerce Act (Queen's Printer, January 1, 2009)
Alberta Agriculture and Rural Development (February 11, 2009)

Edit 29Apr09: updated link to Queen's Printer