using arc tool horizontaly

Perhaps a "Dynamic" symbol or "Art Brush" would do the trick.

• Create a Black-stroked arc
• Rotate 45°
• Drag to "Brush" panel or click "New Brush" icon in Brush Panel.
  • Set the Brush colorize to "Tint" so you can easily change the brush color when used. 
  • The trick to making it start and end on the horizontal axis in which you draw is to include a bounding box double the height of the arc. So do this:
    • Drag the brush art back out of Brush panel. That will give you the initial bounding rectangle.
    • Select the rectangle and double the height using the "Options bar" making sure "Scale from Top" is on.
    • Update the Brush by option-dragging the edited art onto the existing brush.

This could be used as-is, but if you need to change the stroke width it won't work as expected. So…

• Drag the same art into the Symbols panel and choose "Dynamic Symbol".
• Deselect all
• Using the Direct Select tool click the stroke within the Symbol instance that is on the artboard
• Change the color as you wish

If you want the stroke width to remain static when scaling an "Arc Symbol Instance" make sure Preference>General>Scale Lines and Effects is turned OFF.

Here's a link to a file with the brush and symbol made for you.

https://www.dropbox.com/s/suqqqqb3vjz5ozx/arc.ai?dl=0

John,

I was imagining you might wish to create a number of arcs in  row.

It is possible to (more) easily create different size arcs of same proportions, but for a general and more versatile application it may be time for a feature request,

https://illustrator.uservoice.com/

John,

Maybe (far) too far out, but for whichever axis desired (and which version you are using), and presuming you are ShiftClickDragging, how about creating something (such as the undesired arc) and then Effect>Distort&Transform>Transform with the right corner Reference Point and Rotate by something like +/-45 degrees (or +/-135 degrees or something), then in the Appearance palette set New Art Maintains Appearance, then start ShiftClickDragging and enjoy?

You can see all the options available when you double click the Arc tool in the Toolbar.

Try the Pen tool instead. It's the most important tool in the application.