Short answer is no. The reason is because the stat block of a polymorphed creature is completely replaced by the stat block of the beast it is transformed into. As a result, the companion becomes the beast and loses all its companion characteristics.
If a Beastmaster Ranger's companion is subject of a polymorph spell does it retain the proficiency bonuses to AC, hit chance, and damage bonus?
Short answer is no. The reason is because the stat block of a polymorphed creature is completely replaced by the stat block of the beast it is transformed into. As a result, the companion becomes the beast and loses all its companion characteristics.