The right answer is hicieron
Anteayer, ellos hicieron la equitación.
hicieron is the conjugation of the verb hacer used here for the third-person plural (ellos - they) in the preterite tense. In Spanish, the preterite tense (el pretérito) is used to describe actions completed at a point in the past. The verb hacer is an irregular verb. In Tables bellow are indicated the forms of regular verb and some irregular verbs.