Caballito literally means "little horse," but in this context, "a caballito" is a common Spanish idiom that means "piggyback."
However, in the song, the artist uses "a caballito" repeatedly in a more metaphorical and dark way. It refers to the "mono" (monkey) that he has "encima" (on top of him), which is a slang term for drug addiction or a monkey on one's back. So, "a caballito" here implies being carried or ridden by this addiction, highlighting the heavy burden and control it has over his life. It's a powerful and unique use of a common phrase to convey a deep struggle.
Maka's "A Caballito" is a really intense and honest look at the struggle with addiction. The title itself, "A Caballito" [on horseback], refers to the act of injecting drugs, which is a pretty stark image to start with.
The song opens with a heartbreaking question: "ÂżPor quĂ© la vida me mata? No entiendo / Si es tan bonita, Âżpor quĂ© me castiga?" [Why does life kill me? I don't understand / If it's so beautiful, why does it punish me?]. He's grappling with why something so beautiful can also be so cruel, especially when he's "paso los dĂas sufriendo" [spending his days suffering] because of the "maldito mono que yo tengo encima" [damned monkey I have on my back]. He feels like he's destroying his own life, saying "Yo es que me jodo la vida" [I'm just screwing up my life], and it's a cycle he feels trapped in, going "de problema en problema" [from problem to problem]. He even says, "Dentro de mĂ el mismo demonio / Prepara mi velatorio" [Inside me the same demon / Prepares my wake], which is a really powerful and dark image of the internal battle. He's selling his soul "gramito a gramo" [gram by gram], and sees his future as "tan negro / Que para estar asĂ en la calle / Mejor morirme o entrar preso" [so black / That to be like this on the street / Better to die or go to jail]. It's a raw and unfiltered confession of the pain and despair that comes with addiction, and how it makes him feel like he's no longer the man he once was.