Installing htop on FreeBSD requires a particular directory under the linux com­pat­i­bil­i­ty tree be mounted, but I did not want linux com­pat­i­bil­i­ty installed. Luckily, I found a thread about htop on FreeBSD which covered that issue, and another which covered loading the linux kernel module.

Just create /compat/linux/proc, add the requisite entry to /etc/fstab, and # mount linproc. If you have problems, try putting linux_enable="YES" in your /etc/loader.conf.

Done!