Is a woman who misses a prayer considered to have abandoned it intentionally, incurring expiation or some other obligation, given that she did not pray Fajr due to her intense fear of a lizard that prevented her from reaching her prayer area?

You are not permitted to delay prayer beyond its appointed time for any justification. You could have performed the prayer inside the room, as there was a clean spot and sufficient clothing for covering, and a prayer rug is not a requirement. If you had not found clothing or a clean place, you could have asked for them to be brought. Otherwise, performing the prayer without purification at its appointed time is better than delaying it. For Allah, the Almighty, says: ﴿Indeed, prayer has been decreed upon the believers a book [to be observed at] fixed times.﴾ Delaying it intentionally is one of the greatest sins, but since you were ignorant, we hope that no sin will befall you. You must repent to Allah and make up the Fajr prayer immediately if you have not already done so.