I clean horse stalls every week for an older lady, where do I add this? Is this considered "freelance" or "gig" work?

Yes. You will be considered self-employed and file a Schedule C. 

These links will be helpful to you: 

Self-Employed Tax Center  

Tax information for Businesses  

Manage taxes for your gig work 

IRS Gig Economy Tax Center 

To file a Schedule C, here are the steps:  

1. Sign in to your account and select Pick up where you left off  
2. At the right upper corner, in the search box, type in schedule C and Enter   
3. Select Jump to schedule C   
4. Follow prompts, you might be prompted to an upgrade if you have expenses