• dougles,
• douglous,
• doughlas,
• doulas,
• doulglas,
• dougals,
• douglases,
• duglas,
• doglas,
• duglass,
• douglis,
• dougls,
• doublas,
• dourglas,
• douglase,
• douglas,
• douglasss,
• douglasse,
• druglesss,
• houglass

• douglas,
• Douglas Fir.

The plural form of "Douglass" is "Douglasses".

The singular form of the word "douglass" is "douglass". The word "Douglass" is already in its singular form as it is a proper noun referring to a specific person or place.