If Primeape's "Top Drop" attack reveals a LEGEND Pokemon card with no HP printed on it, how much damage does it do (if any)?
No damage is done regardless of which LEGEND half is revealed by Top Drop, because even the half with HP written on it is not considered "a Pokemon" while in the Discard Pile.
Source: PUI Rules Team (2010-02-04), PUI Rules Team (2010-02-18)

Relevant cards

Note: All applicable cards may not be listed below.
PL3_39