\documentclass{article}

\usepackage{multido,xcolor,xfp}
\usepackage{enumitem,xkeyval}

\makeatletter
\define@cmdkey{QA}{question}[\relax]{}
\define@cmdkey{QA}{answer}[\relax]{}
\define@cmdkey{QA}{lines}[4]{}% Default of 4 lines
\define@cmdkey{QA}{points}[1]{}% Default of 1 point
\newif\ifprintanswers

\long\def\MainFrage#1#2\EndMainFrage{%
  \setkeys{QA}
    {question,answer,lines,points,% Set default values
     #1}% Set user-specified values
  \begin{enumerate}[resume]
    \item \cmdKV@QA@question
    \par
    #2
  \end{enumerate}
}
\NewDocumentCommand{\OffeneFrage}{ m }{%
  \setkeys{QA}
    {question,answer,lines,points,% Set default values
     #1}% Set user-specified values
  \ifx\cmdKV@QA@question\relax
    % No question was supplied; do nothing
  \else
    \begin{enumerate}[resume]
      \begin{samepage}
        \item \cmdKV@QA@question% Set question
          \unskip\hspace*{\fill}%
          \ifnum\cmdKV@QA@points>0 % Print points if > 0
            ~\cmdKV@QA@points~P\par
          \fi
          \ifx\cmdKV@QA@answer\relax
            % No answer was supplied
            \multido{}{\cmdKV@QA@lines}{\item[]}% No answer provided; set lines
          \else
            % Answer was supplied
            \ifprintanswers
              \item[] {\color{red}\cmdKV@QA@answer}% Set answer
              \multido{}{\inteval{\cmdKV@QA@lines-1}}{\item[]}% Set additional gap
            \else
              \multido{}{\cmdKV@QA@lines}{\item[] \dotfill}% Set answer lines
            \fi
          \fi
      \end{samepage}
    \end{enumerate}
  \fi
}
\makeatother

%\printanswerstrue

\begin{document}

\MainFrage{
  question={Answer the following questions about animals.}
}

\OffeneFrage{
  question={How many legs does a dog have?},
  lines=2,
  points=5
}

\OffeneFrage{
  answer={A dog has four legs},
  question={How many legs does a dog have?},
  lines=2,
  points=5
}

\OffeneFrage{
  lines=3,
  question={How many legs does a cat have?},
}

\EndMainFrage

\end{document}