http://www.seahorse.org
No. You can't put a regular sized seahorse in a pico.
Dwarfs might work, but their feeding requirements keep most people from trying to keep them.
They also need to be fed fairly heavily, so the tank should be overfiltered, which may be problematic in a pico. You might be able to get away with
every-other-day water changes if you're determined to try though.
Personally I wouldn't try them without doing 2-3 months of research/prep.
B