Why were the animals afraid to enter the farmhouse at first?

The animals were afraid to enter the farmhouse because it had always stood as a symbol of power and oppression. They had chased Mr. Jones and all the humans from the farm the night before and destroyed the devices that held them captive. Now, the following morning they were staring at the final obstacle; the farmhouse where their "master," Mr. Jones, had lived. Once they crossed that threshold, their rebellion would be a success and there would be no turning back.
The fear was perhaps two-fold by entering the farmhouse. On one hand they were saying goodbye to the life they had known, and on the other hand they would be starting a new life with new rules and many unknowns.
Once inside they were overwhelmed with how well humans lived. They made vows and proclamations to not be like the greedy and cruel humans. The farmhouse would become a museum and serve as a reminder of the evils of power.

Following the rebellion in chapter 2, the animals gallop around the boundaries of the farm, making sure every human is expelled. They then rush to the barn and destroy the devices that Jones used to enslave them. They break the reins, bits, and whips in order to destroy the last traces of Mr. Jones's rule.
The next day, the animals once again gallop around the boundaries of the farm to survey their land but stop at the door of the farmhouse. Orwell writes that the animals are initially scared to enter the farmhouse. For their entire lives, the farmhouse was Mr. Jones's residence and symbolizes his terrible authority and tyrannical rule. They have never entered Mr. Jones's home and fear that something malevolent awaits them inside. The enigmatic and ominous nature surrounding the farmhouse frightens the animals, who are apprehensive to enter. However, Napoleon and Snowball walk the animals through the farmhouse, and they are astonished to witness the luxuries inside Mr. Jones's home.