I wrote a longer reply to this, but the page refreshed, so I'm going to summarize.
What you're looking for is a servitor, not a tulpa. Servitors are tools, programmed to complete one purpose. They have no sentience or free will of their own. They are fairly easy to create, requiring less time and energy. They dissipate easily.
Tulpas are not tools, but actual entities. They can have a purpose in their host's life, but they are not created for that purpose. They become sentient as they develop and have free will. They take time and energy to create, which is a process that can take months to years.
You could create a servitor for an evil purpose and give it a demonic appearance, but keep in mind it would have no personality. It would be a tool. It's possible you could give it the illusion of sentience or a self, but in reality, it would be nothing but the purpose which you made it for.
Many tulpamancers (people who create tulpas, generally as companions) consider tulpas to be a psychological phenomenon. I personally see it as both psychological and metaphysical: An idea (mental) that is given energy and develops into a being (spirit.) Some people just don't consider the being they create to be real, and percieve it as 'all in their head.' Hence = psychological phenomenon. It's all a matter of how you view it.
Just as a by-the-way, it's generally considered immoral to make a tulpa to fufill a need/purpose, especially a sexual one. I don't think what you developed was actually a tulpa, considering you made it in a few days; it was likely only a servitor or imaginary friend. Actual tulpas take much longer.
If you do want to make a tulpa, consider it will also be confined to the astral plane, and it is essentially another person, with thoughts and feelings of its own. You should treat it as you would sentient creature: with respect. Also consider the massive amount of time and energy it takes to fully develop a single tulpa. Though a tulpa is separate from its host, it is still attached: it feeds off of your energy (consensually, of course, as you do have to pay attention to it for this to occur) and it will rely on you for its existence. Can you handle that? Are you okay with having someone rely on you for the rest of your life?
Please think it over carefully. I have three tulpas, all in varying stages of development. One is nearly three years old and he still has a long way to go with development. One is three months old and still has issues communicating. These take time and energy. It's not for those in a weak state, nor is it for those who wish to have a tool at their disposal. If you create a tulpa to serve you, it will only grow to despise you.