In Soul Silver i got a sudowoodo and got an egg from it but when it hatched it was another sudowoodo, WHY? (I picked a new game a while after so im just curious)
You need the rock incencse. Give it to Sudowoodo, get the egg, when it hatches it will be a Bonsly . The reason why it did'nt work is because Bonsly came out only in Sinnoh, so they kept the old ways and just added items to get the the new pokemon.
You need to have your Sudowoodo hold a Rock Incense so Bonsly can hatch from the egg.