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At first, she didn't take to that horse.
Literal
She [topic-は] at-first that horse [subject-が] did-not-please.
気に入る ('to take a liking to') literally means 'enter [one's] spirit/mood' — when something 気に入る, it gets into your good graces. The thing you like is the grammatical subject (marked with が), not the object — 馬が気に入った, not 馬を気に入った. The negative 気に入らない reverses it: she didn't take to the horse. 最初 ('at first') is a temporal noun used adverbially, often paired with 後 (later) or 結局 (eventually) to set up a then-and-now contrast.