Brock's Chansey is Brock's 10th Pokémon, which he received as an egg.
The egg hatched revealing the Pokémon to be a Happiny, at one of the Pokémon Centers. As a Happiny, it possessed enormous strength, using that strength to throw Brock into the air when it hatched. Later in the series, Happiny lifted an enormous boulder with one hand and also lifted Ash's Grotle and carried it around Canalave City when it got tired.
While on the boat to Twinleaf Town, Brock's Happiny evolved into Chansey while trying help a Pichu. It is currently unknown if Chansey has kept the strength it had as a Happiny but it knows the move Softboiled which it used to heal the Pokémon on the boat.