Is there an online tool or app to convert XML to DTD?

Many XML development environments such as Oxygen and Stylus Studio are able to generate a DTD from an XML source document.

There's also a freestanding tool I wrote myself many years ago: see http://saxon.sourceforge.net/dtdgen.html

I don't know of any online tools.

Of course these tools are never perfect. For example, if the only values that appear for a particular attribute in the source document are "red", "green", and "blue", the tool has to make a judgement as to whether those are the only legal values. Different tools will produce different results, and you will generally need to do some hand-editing based on your knowledge of the semantics of the data.