2 Rattatas and Ditto

I got another Rattata in Golden State Model Railroad 

Museum in Sacramento, and then 

all of a sudden Rattata was transformed 

by Ditto a Transform Pokémon inside of a Pokeball 

right before my very Eyes.
The last Rattata evolved into Raticate. 

I thought it was another Rattata, 

but it's not, it was a Ditto who transformed into Rattata. 

I got another Rattata outside in Sacramento 

that is regular and it was not transformed by Ditto. 

Rattata and Ditto were normal Type Pokémon.