This is my only 'monster' really!
My four elemental dinosaurs, who all split from the same space monster (doesn't yet have a Toyhouse bio, but it's coming soon). So I either have four, five, or one monster character. Depends on how you view it.
All of their bios link to the series they appear in. Ep 8 provides a backstory (albeit not that detailed) as to how they came to be.
https://toyhou.se/1308611.dr-fire
https://toyhou.se/1431787.dr-water