Richard Dreyfuss (Rodney) was 18 years old at the time of filming (March 18, 1966). Dreyfuss began acting in his youth, and debuted in the TV production In Mama's House, when he was fifteen.
Even though dogs are susceptible to theobromine poisoning, typically from ingestion of chocolate (for some dogs even small amounts can be fatal, especially dark chocolate), for comedic effect, Rodney as a dog enjoys chocolate cake.
Samantha tells Darrin that she thought that they decided on not getting a dog until they got a bigger house. This indicates that the Stephen's intended on upsizing to a bigger house when either family or finances permitted.