Thoughtforms in Buddhism

If you look at this through the Zen aspect of Buddhism and other aspects as well, creating a thoughtform if this is even possible would be a failure to follow instructions. In meditation following the breath and counting is a way to stop running after thought forms. Thought forms are as imperfect as imagination allows. Take someone on psychotropic drugs that sees things they do not see in regular life. Where does that come from is not important to the Buddhist as how to get back to the breath and the beginner's mind. Milarepa is an example of someone who could use black magic to kill people.http://www.thranguhk.org/buddhism/en_milarepa.html

You can read about his black magic, but here is a more relevant detail, the outcome of his karma, remorse.

Seeking the Dharma

Milarepa was filled with remorse for the evil he had done by magic and by hailstorms. While he remained in the lama's service, he asked himself unceasingly and passionately by what means he might practice the true teaching.

The lama replied, "All composite things are impermanent. I have done harm to many beings by evil spells, magic, and hailstorms. You also have accumulated crimes of magic and hailstorms. These crimes will be on my head. Since you are young and have a great faith. You must practice the Dharma yourself and help us all achieve the higher realms and liberation. I will support you with all the provisions you need."

So all of Milarepa's black magic only leads him into a karmic mess that must unwind. So what is the benefit of creating Tulpas or thoughtforms or black magic when it only brings us deeper into the karmic mud?

If you can imagine an enlightened master and he helps you to awaken, then perhaps there is some value in this practice, but be careful because what claims to be a master might only be someone to enslave you in passion. Be very careful. Some sites on the internet actually talked about creating women and men to be lovers with Tulpas. This is as foolish as following the desires in the body. It only leads to suffering.

Ask yourself deeply what is your intention in this pursuit if you must pursue it. If it is only to satisfy desires, then you only sink deeper in the mud.

This book whose title is,

BUDDHISM 
Critical Concepts in Religious Studies 
Edited by Paul Williams 
Volume III 
The Origins and Nature of Mahayana Buddhism; 
Some Mahayana Religious Topics 

says on page 365,

That an adept can employ an advanced meditation technique to create a body made by his or her own mind is an opinion found in ancient Buddhist scripture and elaborated in later treatises. This is a pan-Buddhist notion. The power to transform ones own body into another form and the power to create bodies "made of mind" are two among the ṛddhi, or supernormal powers, explained in the Visuddhimagga.

Few can achieve vikurvana-rddhi; it is a difficult attainment. But Buddhas, Pratyeka-Buddhas and chief disciples are said to attain it automatically when they become Arhats, that is, when they attain liberation from the cycle of rebirth.

You can use the hyperlink above to read more from those pages of that book, or here is a copy as a PDF.

So, to answer your questions:

• Are thoughtforms a common phenomena in Buddhist teaching - that is, do many practitioners (monks or otherwise) have experience in creating thoughtforms? Is it something that a layperson could do, and if so, is it recommended to do so without any other training (for lack of a better word) in Buddhism?

  No, apparently not: it's a "difficult attainment".

• From what I have read so far, Buddhist texts state that these thoughtforms are illusions created by our own minds - but an actual sentient being that can be conversed with is a very powerful illusion. Do Buddhists believe that thoughtforms are a manifestation of the subconscious mind, and therefore a gateway to our own inner workings? If so, is that dangerous?

  They're not something you create in order to have an imaginary friend, which you can talk with and which acts a gateway to your own subconscious. Instead they're something which a Bodhisatva might create as a tool to help liberate someone else:

  The Bodhisattva cultivates rddhi-Abhijñā in order to advance a great many beings toward liberation; without it, relatively few beings can be matured. For this purpose, a Bodhisattva can make for him- or herself the body of a cakravartin, a Sakra, a Buddha, or whatever is required to effect someone's liberation. Such transformations (nirmita) are true and not false, for no dharma has a fixed characteristic

  They're created consciously, not subconsciously:

  Having entered dhyiina, using one or another meditation-object, one should arise from the meditation and contemplate oneself having the form of-for example-a boy. Having done this one should re-enter dhyiina and resolve, "May I be such and such a boy." With resolve, one becomes so. Similarly, in order to exercise manomayii-rddhi, the power of creating a body by mental powers, arising from dhyiina and contemplating the body one should resolve "Let the body be a hollow." The body becomes a hollow, one contemplates another body within one's own, resolves again and there is another body within oneself. One removes it, as a reed from its sheath, and the two are distinct but one is the duplicate of the other. Thus, one uses a progressive visualization exercise, in either case, and the end result is said to be the production of a new body, visible to others.

Beware that I'm reading this from a book: I don't know Tibetan Buddhism (and Wikipedia's article on Tibetan Buddhism suggests that it requires a teacher, an "authentic lineage of transmission" from "a lama who is fully qualified to give it"). The footnotes (on page 379) reference the Visuddhimagga.