I'm afraid a Patapatatchi is an odd generation teenager(1,3,5,7,9 etc) and Decotchi is an even generation adult (2,4,6,8 etc) so one can't evolve into the other.
I have had Decotchi twice. I think both times it evolved from a Hikotchi- that's the little guy with rocket boosters!
It is a good care adult- coming from the same level as Mimitchi/Mametchi etc.
However I can never work out what specific things determine you get a certain character
