A previously convicted New Jersey sex offender has been arrested again after authorities said he had videos of himself performing sex acts on a horse and child pornography. Mitchell B. Richtman, 31, was caught after the National Center for Missing and Exploited Children tipped off the Mercer County Prosecutor’s Office, officials said. Investigators obtained a search warrant and found the videos on electronic devices in Richtman’s East Windsor home, the prosecutor’s office said Monday. Forensic previews revealed child pornography depicting children as young as 5. Richtman was charged Thursday w…