[Tex/LaTex] Are there placeholders in LaTeX

Question

Are there placeholders in LaTeX of the form:

A horse is \placeholder able to fly.

\placeholdertext{not}

which should produce an output like:

A horse is not able to fly.

So I look for an analogon of \footnotemark and \footnotetext that works with arbitrary content.

I would like to use that for example in combination with marginpar, when I want to explain some simple term in a paragraph with a complex tikz picture on the side:

Some long paragraph in which I mention some term\marginpar{\placeholder} that I want to explain on the side.

\placeholdertext{
    \begin{tikzpicture}
    % Picture for a better understanding of the used term.
    \end{tikzpicture}
}

Another paragraph with some term\marginpar{\placeholder} that I want to explain.

\placeholdertext{
    Some informative text...
}

Answer 1

You can use the todonotes package to insert not in the margin:

\documentclass{article}
\usepackage{todonotes}

\begin{document}
A horse is \todo{not} able to fly.
\end{document}