what is "rocking horse people eat marshmallow pies"?
I read it in this sentence:
One artist describes their room as a "flip-book in motion" while another describes hers as "The glorious, technicolor-dream-coat room where rocking horse people eat marshmallow pies. "

Question

what is "rocking horse people eat marshmallow pies"?
I read it in this sentence: 
One artist describes their room as a "flip-book in motion" while another describes hers as "The glorious, technicolor-dream-coat room where rocking horse people eat marshmallow pies. "

Answer

Hi Huanmer,
The guy is under the influence of LSD = Lysergic acid diethylamide  probably *a drug* and hallucinating. 
This drug has  unusual psychological effects, which include visuals of colored patterns behind the eyes in the mind, a sense of time distorting, and crawling geometric patterns,  made it one of the most widely known psychedelic drugs. It has been used mainly as a recreational drug at the end of the 50's and in the 60's . So what he sees is a LSD trip ;) it is called so.

Answer

It's a reference to the Beatles' song 'Lucy in the Sky with Diamonds.'

It is a nonsense phrase, or more accurately it's describing a fantastical place with crazy nonsensical things. 

But... I think in your sentence, becasue it's referencing the song, which is talking about psychedlic experience, I think the sentence is saying the artist's work is influenced by psychedelic experience, or it looks like it.