Goal: With the .com hosted at Namecheap, point to main domain on SiteGround and to sub-domains on HostGator.

Put a Subdomain on a Different Server by CSS-Tricks “…you just need to a new A record for the domain at your domain registrant.”

Namecheap

Choose Namecheap BasicDNS from the drop-down menu. Source

Since I had BigBlueTaco.com’s Namecheap DNS set to ns1.siteground222.com and ns2.siteground222.com, I had to change the Namecheap Nameservers setting from Custom DNS to Namecheap BasicDNS.

DNS changes may take up to 48 hours.

Host records setup

If your domain is pointed to our BasicDNS, BackupDNS (a legacy option), PremiumDNS or FreeDNS, you can set up A, AAAA, CNAME, NS, SRV, TXT, URL Redirect, MX, MXE, CAA records from Namecheap’s side. Source

Siteground: How do I set up host records for a domain?

The Host Records section is available only for the domains using Namecheap BasicDNS, BackupDNS (a legacy option) or PremiumDNS. To set up host records for your domain, use the following instructions:

A (Address): Allows you to associate a host with an IPv4 address. The IP address that you use does not have to be on your network. For example, you can have the host record for www pointed to 207.46.130.14 (the address for the Microsoft website). Source

CNAME (Alias): Is used to associate a host name with another host. The host that you wish to point to does not have to be on your network. Please don’t set up CNAME for a naked domain (@ hostname) since it may affect the operation of the domain’s MX records and, consequently, the email service. When you use a domain in the address field, you must add a trailing period (“.”). Unless you do so, the name server will add your domain to the end of the given domain. Source