Horsea and Cascoon are extremely rare, but Horsea can be found on Misty's map, Drake's map, Wallace's map, and the Underwater map. But you can only get it like 1/100 chance, maybe even less. Cascoon, you have to get a Wurmple and hope with all your strength that it will be a Cascoon. Usually it is a Silcoon, but you can still get it hopefully.
*Edit*
Ermm... Horsea and Cascoon are really common and its not a 1% Chance
-New009
HOW GET A HORSEA ON POKEMONINDIGO.COM
You can find one in a room or pokemon locator
You can catch Horseas around level 10-13 in The Underwater Zone (Indigo World Map).
A Horsea will be found if you get a good rod and look for a lake or river.
whell horsea can be found in the misty map but itsrealy REALY rear and it can b found in Underwater map in INDIGO WORLD......but you can allways trade, but you need a groudon,palkia ho-oh,suicune or eny other cool pokemon.....ok you can find horsea at misty map and you can find him on the underwater map also.
Cascoon is a Bug type pokemon.
Fish on routes 132 133 134 with a super road they are very commen so they wont take long to find #
You can't get horsea in xd.
Horsea is a Kanto region pokemon.
A easy way to train Your Cascoon is as follows. First out it at the first spot of your party. Then go to a place with strong Pokemon. Send out your Cascoon then swap it out with your strongest. Defeat the other Pokemon and your Cascoon will get a lot of exp. points.
you get horsea on route 226
You can find Cascoon in Pokemon Platinum at Route 205 North, Eterna Forest.