There seem to be multiple points to this question: firstly which Stat Block is the official one for any given campaign, and how this affects the Druid's Wild Shape ability.
The Stat Block Officialness Question
The DM decides which stat block to use.
Additional Info and Suggestions:
If you're currently running Out of the Abyss [or any other module with campaign specific stat blocks], I would recommend using the stat block included with the module, as the differing abilities of versions in other source books might change the challenge level or interact with other creatures/situations in unexpected or undesireable ways. The theory here is that the stat blocks for a module have been play tested to work well in that module.
Otherwise, barring DM preference, I'd use the most recent official version, which in this case is MToF.
If homebrewing is an option, the DM might also choose to merge the stat blocks, taking attributes from each.
The Wild Shape Question
The key text of Wild Shape [as copied from the SRD] states:
Starting at 2nd level, you can use your action to
magically assume the shape of a beast that you have
seen before. You can use this feature twice. You
regain expended uses when you finish a short or
This means that two conditions must be met for a Druid to use Wild Shape to transform into a Steeder.
- The DM must be using a monster stat block that defines the Steeder as a beast.
- The player character must have seen a Steeder at least once during the course of the campaign or in their backstory.
If both conditions are met, then yes, the Druid can Wild Shape into a Steeder.