How I Met Your Mother had a brilliant Easter egg confirming that Marshall found the Loch Ness Monster. The nod to the creature was shown through a flash-forward scene in the sitcom's third season. Here's a breakdown of Marshall's interest in creatures from folklore and how it played into the series.
Marshall Eriksen's (Jason Segel) obsession with mysterious creatures and supernatural beings was confirmed in season 1, episode 7, "Matchmaker." Marshall and Robin (Cobie Smulders) debate about their beliefs on the supernatural after the "cockamouse" incident. Robin compared the cockamouse to myths such as the Loch Ness Monster which greatly offended Marshall. He has always had an interest in strange phenomena such as the Loch Ness, Bigfoot, and Area 51. Marshall even referred to the Loch Ness Monster as "Nessie" as he thought the gentle creature shouldn't be classified as a monster. This would not be the last time Nessie would be featured on How I Met Your Mother.
After Marshall and Lily (Alyson Hannigan) got married in the season 2 finale, the duo went on a honeymoon to Scotland in order to look for Nessie. They were initially only supposed to dedicate one day to hunting Nessie but Marshall's obsession hijacked their honeymoon. The journey was later deemed successful because a framed picture of Nessie could be seen in the season 3 episode, "We're Not From Here," revealing that Marshall did indeed find the Loch Ness Monster. As shown in Future Marshall's office, there was a newspaper clipping hanging on the wall with the headline "NYC Lawyer Captures Nessie."
The specific episode centered on Lily and Marshall's struggles with writing their death notes. In case either of them were to pass away, they wanted their spouse to have useful information and a lasting letter. Marshall forgot so he poured his heart out in his letter but later found that Lily's letter was a lot less touching. She agreed to write a new one but made Marshall promise to not open it until she was dead. In a flash-forward scene set in 2029, future Marshall broke the promise and opened the letter early realizing that it was another fake. The couple in the future then argued about the death notes again but it also confirmed Marshall's achievement with the Nessie Easter egg on the wall.
The actual Nessie hunt was featured through "Lily and Marshall's Honeymoon Videos," a set of scenes on the How I Met Your Mother season 3 DVD that chronicled the couple's Scotland trip. The duo planned to spend one day at Loch Ness but as the videos displaying the days went by, Marshall and Lily were still there. Marshall refused to leave Loch Ness without seeing Nessie. At one point, he claimed to see the creature coming towards them so he ran away and crashed into a tree.
As the days went by, Lily grew angrier as they missed out on the rest of their trip. By day 10, Lily had absolutely had it with her husband. She took the car and drove away, leaving Marshall chasing her by the side of Loch Ness. He presumably caught up with her because in the next video, the couple had decided to stay in Scotland for an extra week but Lily got to decide on their itinerary. According to the newspaper, Marshall must have caught Nessie on camera during some point of their trip. If only How I Met Your Mother captured Robin's reaction to the creature's true existence.