The best open source alternative to Termius is PuTTY, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Termius and 18 is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Termius are Remmina (Free, Open Source), mRemoteNG (Free, Open Source), Terminator (Free, Open Source) and KiTTY (Free, Open Source).
PuTTY is a free and open-source terminal emulator, serial console and network file transfer application. It supports several network protocols, including SCP, SSH, Telnet, rlogin, and raw socket connection. It can also connect to a serial port.
Remmina is a remote desktop client written in GTK+. Remmina primarily targets system administrators and travelers that need to work with computers remotely in front of either large monitors or tiny netbooks.
Terminator enables the user to run and manage multiple terminal emulators side-by-side in the same window. The main focus is arranging terminals in a grid. (Tabs is the most common default method, which Terminator also supports.
ConnectBot is a Secure Shell client for the Android platform. Its ultimate goal is to create a secure connection through which you can use a shell on a remote machine and transfer files back and forth to your phone.