How many times has Pacquiao been underdog?

Probably because he cam up from Flyweight.
People were underestimating his size and ability.

 

I've read countless nonsense about how Horn is going to beat him in the lead up...

 

Pacquiao was actually the betting favourite for this (albeit narrowly) and was a 2/9 favourite against Margarito.

Here is a link to a page detailing all of Manny Pacquiao's fight odds since the Oscar Larios fight (the fight before his decider with an Erik Morales whom he pretty much ruined in their second fight) to present. As you'll see, he has only been installed as the underdog twice in this entire run -- vs Oscar De La Hoya, and vs Floyd Mayweather Jr:

http://www.oddsshark.com/boxing/manny-pacquiao-betting-odds

In addition to this, Marco Antonio Barrera was a 1/4 favourite for Pacquiao vs Barrera I:

http://www.boxingnewsonline.net/on-...ing-world-by-thrashing-marco-antonio-barrera/

 

Kudos for being the greatest overachiever. Give him credit for Barrera. Ultimately, he failed against the guy he needed to beat to be the best of his era.

 

As far as I can remember, I think the oddsmakers also had the first fight with Morales as a pick 'em, maybe with the odds ever-so-slightly favouring Morales towards fight night.

 

Those don't exist lol they stopped after Floyd beat him