In my hardcover of Out of the Abyss and on the source of it here :https://www.dndbeyond.com/sources/oota/appendix-c-creatures#MiscellaneousCreatures Steeders are listed as beasts, however in the monster section, they are listed as monstrosities.  

https://www.dndbeyond.com/monsters/female-steeder

https://www.dndbeyond.com/monsters/male-steeder

Unless there has been an additional errata to Out of the Abyss they should still be beasts, And its odd because I know they were listed as beasts on here before because it showed up when looking for potential ranger pets.

Not an Errata on Out of the Abyss, but Steeders got reprinted in Mordenkainen's Tome of Foes: minor changes, but they are now monstrosities.

OK sorry for bother.