fifth wheel of the coach

fifth wheel of the coach — (нужен как) собаке пятая нога (телеге пятое колесо)

see also
[как рыбе зонтик]
[fish needs a bicycle]

a fifth/third wheel (an unneeded, extra person or thing; someone who is in a situation where they are not really needed or are ignored by other people) — лишний, не пришей кобыле хвост, третий лишний

feel like a fifth / third wheel — чувствовать себя (третьим) лишним, ненужным, ~ чувствовать себя не пришей кобыле хвост

I don't have a role in the office any more—I feel like a fifth wheel. (Cambridge Idioms Dictionary)
He was the only one without a date, so he felt like a fifth wheel.

Origin: This is a proverb that was first used in France in the 16th century. A unicycle has one wheel; a bicycle has two wheels; a tricycle has 3 wheels. The fifth whelel is a surplus thing, good for nothing. In the same way, if 2 couple are going out on a double date, and extra person tags along could be called a "fifth wheel".