There is a desire not only to be submissive but to be shaped into the role of a master and mistress' pet. One fastasy starts out with being abducted. I eventually awaken fully to discover that not only am I naked and caged, but I have breasts and appear very feminine...
Over time the pet is trained to be obedient, following commands to sit, stay, or fetch...
...perhaps I will play the ponygurl for the month...
And last but not least, the role of the cow which would require milking !