Which pokemon is number 67 on the pokedex in pokemon diamond?

In the Sinnoh Dex? Buneary. Buneary are common in Platinum at Eterna Forest. Diamond/Pearl owners can also find one there, but not as common.

In the National Dex, #067 is Machoke. Machoke can be found on Route 226. AGAIN, more common in Platinum, 25% as opposed of Diamond/Pearl's 10%.

Hope that helps!