Is it possible to have a one line each block in Ruby?

Yes, you can write:

cats.each { |cat| cat.name }

Or simply:

cats.each(&:name)

Note that Enumerable#each returns the same object you are iterating over (here cats), so you should only use it if you are performing some kind of side-effect within the block. Most likely, you wanted to get the cat names, in that case use Enumerable#map instead:

cat_names = cats.map(&:name)