Hengeyokai shapeshifting is Alternate Form (which used to function as polymorph self in 3.0) so it gets updated to the 3.5 Alternate Form ability, not 3.5 Polymorph.
Hengeyokai text specifically says :
This supernatural ability functions like the polymorph other spell...
Alternate form doesn't get this kind of specification normally. (Like for True Dragon alternate form for example where no mention of this spell exist)