The Portuguese version of the Wikipedia article probably gives the most realistic reasons why and where the book was banned:
Ferdinando, o Touro (Portuguese)
Even then, supporters of dictator Francisco Franco classified it as a pacifist book, being banned in many countries that adopted fascist models of government. On the other hand, due to the prohibition by these regimes, the little novel was promoted as a leftist ideology book.
The English version of the Wikipedia article uses a quote that cannot be varified. The original article it uses as it source, does not, itself, contain any information where this quote comes from. The Spanish and Catalan versions are directly translated from the English version. The other language versions do not use this quote at all.
The Story of Ferdinand
It was banned in many countries, including in Spain (where it remained banned until after Franco's death). In Nazi Germany, Adolf Hitler ordered the book burned (as "degenerate democratic propaganda"),
The given source () is the The Washington Post article by Michael Patrick Hearn from November 9, 1986, where the line
In Nazi Germany, Hitler burned the translation for being "degenerate democratic propaganda."
is taken from.
The original article from 1986 gives no source for this claim.
Searching for "degenerate democratic propaganda" , search results will be found from:
- 1986, 2004, 2005, 2009, 2013 to 2019
in different varieties:
Franklin D. Roosevelt Presidential Library:
Adolf Hitler labeled the book “degenerate democratic propaganda.” He and Spanish dictator Francisco Franco viewed it as anti-Franco and banned it.
Julie (Canada)’s review of The Story of Ferdinand:
It was banned in Nazi Germany (by none other than Little Adolf) for being "degenerate democratic propaganda."
None of which give any sources for the claim.
Since the Michael Patrick Hearn article seems to be the 'Claim Zero' (no results found before 1986) and that Adolf Hitler being the most exposed person in the last 100 years, not finding a reference to the 'quote' before 1986 should be considered (at least) as odd.
Conclusion: Quote cannot be varified as authentic.