In Latin - the language from whence the word came - 'omnibus' is already plural. It is the dative plural form of 'omnis' and means 'for all'. In English, of course, we double-pluralise it when we say 'omnibuses'.
'Termini' is generally pronounced ter - min - eye rather than ter - min -ee.
Terminus is a Latin word that we sometimes use. The plural is termini. Terminal is an English derivation from that Latin word. The plural is terminals.