Which Machines can I mount?

There are only a handful of mountable Machines. Based on this article, it looks like the only mountable ones are:

• Striders
• Broadheads
• Chargers

None of the others are able to be mounted, but will instead become friendly and attack hostile Machines on sight.

It's worth noting that your mount is not permanent by default, however, there is a perk that enables you to have a permanent mount that you can call at any time if you don't actually have an active mount, as mentioned in this guide:

But there is an easy way to unlock permanent mounts. They can also help you in battle! You can call a mount whenever you want, how often you want. Here’s how to do it:

• In the Skill Menu buy “Call Mount +” (Forager Skill Tree).