'Dance Moms' Suspect -- Criminal Record Includes Lindsay Lohan Obsession

The guy busted for sending child porn to one of the "Dance Moms" kids had violent fantasies about Lindsay Lohan and served time in prison and a mental hospital ... TMZ has learned.

According to court docs we've obtained ... Phoenix Sundown was arrested back in 2007 for assaulting his 18-year-old girlfriend. He was 29 at the time. Sundown got 3 years in a state pen, but was eventually diagnosed with a mental disorder and transferred to a prison hospital.

During counseling there ... doctors discovered Sundown had several photos of women with red pen and wax markings on their breasts and crotches. One collage showed a Lindsay Lohan pic with trails of red dripping on her body, and Sundown holding a long knife over her.

He also copped to writing to Spin magazine about his sexual fantasies with several other female celebs, but said he never contacted them directly.