(Examples below are illustrative only and not real domains.)
Imagine I have turtles.com and the site has been up for some time. Now imagine that I have reptiles.com.
Would it be "bad" (for SEO, Google, etc.) to redirect turtles.com to reptiles.com/turtles, where /turtles is just a subpage, not a subfolder/subdomain?
Also, what would be the best way to do this?
A 301 redirect would pass the authority of turtles.com to /turtles and also benefit the new domain as a whole. The question of "bad" is whether you are missing out on any SEO from the old page and whether there is an opportunity cost. A 301 redirect will transfer the authority from the old page and so it would not be "bad".
Since /turtles is a subpage, every page on the old website would redirected to the one page. Make sure that if someone visits turtles.com/feeding-advice, /turtles would somehow still be relevant to them. It may be that you making a few more /turtles pages with separate redirects is a good idea.
The best way to do the redirect is to create a 301 permanant redirect rule. On an Apache server, you can create a rule in the .htaccess file, however, that requires you maintain hosting. Check with your domain name provider whether you can manage redirects from there. I use Cloudflare to manage my DNS and you can set pagerules to redirect the website without needing a hosting account.
