Well I finally found a translated copy of Buckbaird's first appearance in the GeGeGe Kitarou anime (not the manga, sadly). Interestingly (for me at least), the fansubber translated his name as Bayard. Personally, I'm too used to "baird" now, but that's just the French spelling. In other countries it's spelled Baiard or similar. A quick google showed that Bayard was not only a surname, but also the name of a legendary magical horse. ... The modern definition is a fearless and chivalrous soldier. As for the Buck part, I can't help but wonder if it was inspired by Buck Rogers, which came out in many years before the GeGeGe Kitarou manga. It could also come from the slang "buck", an old term for a black male (Buckbaird is a giant, black eyeball with frizzy hair, if you look at him with the right mindset). There is also the name Buck Byard, but the last name is pronounced differently, I think. I haven't found anything in the Cthulhu Mythos that comes close to Buckbaird, either.
But anyway, I can say more about who Buckbaird was. According to the original anime (1968), Buckbaird was head of the monsters in the West (Europe), presiding over the likes of Dracula's descendants, a witch, and the Wolfman. He has the power to make people stupid. He turns London police into blubbering idiots. Later he numbs Ratman's brain so he'll tell the truth.
When London police had the monsters' party hall (because Western monsters love to party) demolished after an incident with Buckbaird one night, the monsters waged war against humans. Sick of being surrounded by humans, Buckbaird vows to travel to Japan and turn the entire island into a haven for monsters to party. They establish their base of operations on Kikai Island ("Demon Realm Island", yes it's a real island off of Japan). To counter them and save Japan, Kitarou tasks Ratman with gathering numerous youkai (including a monster from Mizuki's "Akuma-kun"), then Kitarou weeds out the weak-hearted and is left with those GeGeGe Kitarou fans know as his best friends (Ittan Momen, Nurakabe, Sunakake Baba, and Konaki Jiji). Ittan Momen tries to take out the witch, who was sent to scout for trespassers, but she stabs a nail through his heart then kidnaps Ratman. Kitarou goes after the monsters alone to save Ratman, but Ratman betrays him to them. A small scuffle ensues between the monsters and the youkai, resulting in Nurikabe's defeat. Kitarou returns and the youkai take up camp with the remaining humans on the island in a cave. Buckbaird and Ratman invade the cave but Kitarou fends Buckbaird off with his Hair Needle attack, although he faints from overexertion. Sunakake Baba and Konaki Jiji waylay the monsters and defeat Dracula's underlings, but then Dracula sucks Konaki Jiji's blood and his surviving underlings suck Sunakake Baba's blood. Buckbaird decides to use Kitarou as his Japanese ambassador and sends Dracula to go lure Kitarou out of the cave. While everyone's distracted, Kitarou's dad steals the witch's broom, which Ratman used as a clothesline for Kitarou's vest, and flies back to the cave on the broom; the witch gets her broom back but Kitarou's dad manages to escape with the vest. Back at the cave, Kitarou tricks Dracula into turning his back on him and strangles him. When Buckbaird learns the humans went into hiding, he orders the witch and Ratman to guard the only source of fresh water on the island. While Kitarou is on his way to fetch water for the humans, he finds his dad and gets his vest back, but right then Buckbaird finds him and uses his powers on Kitarou to brainwash him. Under Buckbaird's control, Kitarou betrays the humans' hiding spot to the witch and Ratman, but Kitarou's dad uses Kitarou's vest to get to the humans; the vest wraps itself around the witch's head and suffocates her, forcing Ratman to flee. When a storm approaches the island, the lighthouse keeper and his son head to the lighthouse to ignite the flame, but they're spotted by Buckbaird's remaining underlings; the dad leads the vampires on a goose chase while the son makes his way to the lighthouse. A small fishing boat approaches the island and is hijacked by Buckbaird and Kitarou. As Kitarou's dad tries to return the vest to Kitarou and snap him out of Buckbaird's spell, the lighthouse light shines in Buckbaird's eye and blinds him, thus freeing Kitarou from the spell. Buckbaird tries to regain control of Kitarou but the lighthouse blinds him again on the next rotation. Ktarou kicks Buckbaird square in the eye, defeating him. He then takes care of Buckbaird's underlings, but unfortunately is unable to reach the lighthouse keeper in time to save him from the last of Buckbaird's minions. On their way back home, Kitarou mourns the deaths of the youkai that joined him in the Great Youkai War.