Dueling Games: Monster Rancher was formally accepted as one of Sony's counters to Nintendo's Pokémon, even going so far as to include Playstation references in the animated series. However, it stopped being a Playstation-exclusive series with the release of Monster Rancher Advance for Gameboy Advance.
The original Monster Rancher never came out in PAL regions—instead, its sequel, Monster Rancher 2, came out first with the number dropped. This caused certain aspects of the anime and the spinoff games to become cases of Marth Debuted in "Smash Bros."—the Dino species were MR1 only.
Similarly, the original DS game never came out in English. English-speaking markets got DS 2 instead, with the similarly-dropped number.
Urban Legend of Zelda: Monster Rancher Metropolis, a well-known site for MR data and gameplay tips, got several things wrong that snowballed into rumors only debunked in 2018.
The Fairy Hare was said to have a boosted Speed stat due to a glitch in early versions of the game, but it actually had a boosted Guts regeneration.
The requirements to get a Beaclon, while strict, aren't as bad as many thought, and there is no random element if you do everything right.
Missing Episode: The episodes "Tiger's Battle with Destiny" and "The Battle with the Big Bad Four" only aired once, and were taken out of syndicated airings until the series was released on DVD.
Network to the Rescue: The syndicated BKN broadcasted the anime early weekday mornings, and only reran (repeatedly) the first season. Fox Kids not only ran the first season on its Saturday morning line-up, but also broadcasted the second season. Unfortunately, they only ran the second season in full once.
Then, out of nowhere in 2002, it was quietly picked up by Kids WB, but only for its weekday block. Regardless, it only aired episodes of the first two seasons.
Genki Sakura's voice actor changed from Saffron Henderson to Andrew Francis. The reason for this is the former's wedding, which took place sometime in January or February 2001, conflicted with her voice acting commitments.
Golem's voice actor changed midway through season 2 from Richard Newman to Doc Harris.
Screwed by the Network: The first season was rerun into the ground, and the second season was only aired in full once, with two episodes swiftly being removed from rotation afterward: "Tiger's Battle with Destiny" and "Battle with the Big Bad Four": the final confrontations with Gray Wolf and Naga. The third season was dubbed but never aired in the US.