What is the reason for imagining a snake coming out from under the bed upon waking up and being terrified by it, and what is the solution?

What the questioner experiences may be the intimidation of devils, and the best remedy for that is to seek refuge in Allah, to supplicate frequently, to fear Allah, to maintain ritual purity and prayer at their prescribed times, to recite the morning and evening remembrances and the remembrances before sleep, to constantly recite Ayat al-Kursi, especially before sleeping, and to keep away from places of amusement and heedlessness.