Though it isn't one, daktari behaves like an M/Wa noun. "The doctor s/he is here". Therefore the third person singular pronoun a- is used. This pronoun has an irregularity: when using locative suffixes (-po, -ko, and -mo) it changes from a- to yu-.
As for why it is -po and not -ko is because the doctor is here, which is a definite location.