I think it's a Croton, one of the Codiaeum variegata types, of which there are many in various colour ranges with narrow or broader leaves. It could even be the variety 'Goldfinger', despite its having less brightly variegated leaves, that could be a result of insufficient sun/bright daylight. It requires bright light, preferably with some sun, and it prefers humid air - mist two or three times a week to prevent spider mite, to which they are prone when grown as houseplants. More info here: Growing Croton Plants Indoors.
It looks relatively healthy, although that bare stem either means it was a standard originally, or its lost all its lower leaves. It's probably time to separate the plants in the pot to give the Croton more root room though.
Note: Rubber tree usually refers to Ficus elastica, although many common names are used for more than one plant.