I have fought so much with this that, now that I could make it run, I want to share it for anyone who also had a problem.
This guide is intented using Unity 5.6.3f1 It should work with other versions, but I only tested it with that exact version.
This was done in a clean boot of Solus.
First step, download Unity for Linux.
Be sure to download the SH file, not the deb
Install the following libraries:
sudo eopkg it libglu libglu-32bit libpqxx gconf gconf-32bit gtk2-engine-murrine gtk2-engine-murrine-32bit gnome-sharp gtk-sharp libgcc-32bit libgnomeui libice libice-32bit libsm-32bit libstdc++ libstdc++-32bit libxcb libxcb-32bit nodejs libpqxx-devel libpng12 libpng12-32bit
Once the file is downloaded and you have the libraries installed, follow this steps:
cd folder-where-unity-was-downloaded
./unity-editor-installer-5.6.3xf1Linux.sh
# if /opt/ folder does not exist we create it
sudo mkdir /opt
# we copy unity to the opt folder
sudo mv unity-editor-5.6.3xf1Linux/ /opt/Unity
# we travel to the applications folder and create the desktop entry
cd /usr/share/applications/
sudo ln -s /opt/Unity/unity-editor.desktop unity-editor.desktop
Done, now it should appear on your search bar when you write “Unity”.