When creating a content type I would like to have a field type that could be used to call a rest service or make a db call to an external database. Does this exist?
To the best of my knowledge, no, there isn't a module out there to do this. I have seen something similar done in the past though, where a node would have an id of some sort provided, and a module would connect to an external service to load in information for the node.
I suspect you might want to think about writing a custom module to do this. If you haven't taken a look at https://drupal.org/developing/modules yet you might want to. Just remember when modifying functionality of Drupal you should never hack the core, but create/use a module instead.
I think what your gonna want to do is architect a way to know what you need to pull and from where, then use hook_node_view to change/add the data you pull from your DB/external service (https://api.drupal.org/api/drupal/modules!node!node.api.php/function/hook_node_view/7)