Competition for mates in sea horses
Second paper is an article in science Daily and have a pretty decent source from Texas A&amp;M University. It's much better than the first as it is un-sourced post on yahoo answers.

Source: 
"Male Seahorses Are Nature's Mr. Mom, Researchers Say." ScienceDaily. ScienceDaily, 2 May 2008.

Why male seahorses gets pregnant?

Genetic answer:

May be because only $MHII\beta$ gene present in the male pouch which carries the babies.

Source and more on this topic:

The impact of sex-role reversal on the diversity of the major histocompatibility complex: Insights from the seahorse (Hippocampus abdominalis)