Thoughtforms in Buddhism I've recently discovered the concept of Tulpa or "thoughtforms", and I have some 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?

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?

For those with personal experience with thoughtforms:

Are your thoughtforms permanent?

Knowing that they are illusions, how "real" do they feel? Are their voices different to that of your mind's voice? Do all of your interactions with the thoughform take place in the mind, or have you experienced them in a physical way?

Have you ever felt attached to a thoughtform, and has being a host for one ever cause you emotional strain?

Does your thoughtform take on a physical shape or image, and if so, was this a conscious decision by you, or did the Tulpa decide it's own form?
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.