He objected to what he felt was a decline in the design and detail of official Royal Mail stamps and the demise of posted letters because of email.
Mr McDonagh said: "When I started I wanted them to be deliberately silly, so I had a fake moustache or beard or eye patch, that was very obviously drawn on very crudely.
"I started it as a bit of a protest. It seemed as if stamps were disappearing due to everyone going online all the time.
"The Queen's head, it seemed to me, was going to disappear from stamps and be replaced with lots of other images and I felt I had to act.
"I just kept going and it has become more and more farcical. It's gone undetected for so long now it is just silly."
Angus has created 50 individual stamp designs and printed them on his home computer and stuck them to envelopes with glue.
All stamps have a fake value of 50c - his own invented currency - and are franked with a location mark by Angus before he posts them.
He makes special edition stamps, but instead of commemorating real life events they celebrate his own fantasy occasions such as 'Upside Down Day'.
One stamp is simply a black and white snap of him as a six-year-old boy, while another is a sweetly-posed portrait of him and partner Jo Purvis, 52.
He has successfully sent over 100 letters to France, Hong Kong, Australia, Canada and Italy - as well as all over the UK - and only one has ever been detected as a counterfeit.
Angus, of Bridgwater, Somerset, claims he never meant to evade payment and has even tried to send Royal Mail a number of cheques for the total costs, but they were all returned.
He said: "I'm actually a great fan of Royal Mail. I think the local postmen in our rural community are very important people who do a great job.
"But Royal Mail has been sold for many billions of pounds when it seems to me that the system in place just doesn't work.
"I have records of everything I have sent with my own stamps.
"The critical thing is I have never intended to defraud the Post Office from any money.
"My solicitor has sent a few cheques for around £200 with a letter saying it is for unpaid postage, but they are always returned."
The Royal Mail has confirmed they are investigating.
A spokeswoman said: "We would like to make it clear that it is a crime to create or use counterfeit stamps.
"We will take the necessary steps to protect the integrity of stamps on behalf of the 29 million households and businesses we are honoured to serve."