"Macho" alone means the same as "mulo": a male mule, for which the proper term in English is a horse mule (as opposed to a mare mule, which is a "mula", a female mule. Strictly speaking, the words "mulo" and "mula" in Spanish, which mean "hijo/a de asno y yegua o de caballo y burra", correspond to both "mule" (the offspring of a male donkey and a female horse) and to the rarer "hinny" (the offspring of a male horse and a female donkey), so in theory "macho" or "mulo" could be a horse hinny, but conventionally "mule" covers both in English too.
http://en.wikipedia.org/wiki/Mule
http://es.wikipedia.org/wiki/Mulo

For this meaning of "macho", see the DRAE entry:

"macho
1. m. Animal del sexo masculino.
2. m. mulo (‖ animal). [...]"
http://buscon.rae.es/draeI/SrvltGUIBusUsual?TIPO_HTML=2&TIPO...

And so it was in historical times; the second definition of "macho" in the Diccionario de autoridades (1734) is: "Por antonomásia se entiende el hijo de caballo y burra, ù de yegua y asno".

The "macho manso" is contrasted with the "macho cerrero" in historical Mexican documents; for example:

"Nuestra Señora del Rosario, 3 088p. (Valor de 564 reses de fierro arriba, 60 caballos, 4 machos mansos, 9 mulas y machos cerreros, 12 potros brutos y un burro padre)"
"El gran Michoacán en 1791: Sociedad e ingreso eclesiástico en una diócesis novohispana", p. 86
http://historiayreligion.com/wp-content/uploads/2012/02/El-g...

A "macho cerrero" was an untamed mule:

"Se llamaba macho cerrero al mulo sin amansar"
Carmela Velázquez, Diccionario de términos coloniales, p. 79

So a "macho manso" is a tame mule, specifically a tame horse mule (male mule).

I have found no evidence that "manso" means "castrated", and I don't believe it does. Nor does "amansar" mean "castrate"; it means "tame" or "domesticate".

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Note added at 57 mins (2012-04-20 17:35:32 GMT)
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On the term "horse mule":
"Male Mule The correct name for a male mule is a Horse Mule, but
they are sometimes referred to as a John or Jack Mule."
http://www.thedonkeysanctuary.org.uk/files/donkeys/MulesHinn...

However, reading a bit further on this, I think I must take back my last comment above: I think "manso"/"amansar" quite probably do refer to castration or gelding. The document just quoted goes on to say:

"Male mules and male hinnies are anatomically normal. They are extremely difficult to handle and should be castrated, to help with their temperament and make them safer to manage."

So it may very well be that "manso" means "docile", because castrated", and that a "macho cerrero" was uncastrated. I would change my answer to docile horse mules; you could actually say "castrated" or "gelded", but I would leave it euphemistic, to match the Spanish. But they are mules, no question.

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Note added at 4 hrs (2012-04-20 21:02:43 GMT)
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I thought the evidence that "macho" means "mulo" was pretty clear, but in the light of Rosa's disagreement, let me add a couple more references. There are many from Spain, but these are from Mexico:

"La avenida Macheros es una de las vías más antiguas de Ensenada. Su nombre nos da indicio de ello. Esta singular denominación “macheros” se le dio a la avenida por localizarse en este lugar, por la altura de la calle Cuarta, un solar en el cual había una pensión para caballos y machos, es decir mulas. El diccionario de la Real Academia Española define el término machero como corral para machos (mulos)."
http://www.elvigia.net/noticia/vialidad-con-historia

"Macho o Mulo. Cruce de yegua y burro o de burra y caballo."
http://mexicocriollo.mforos.com/1530527/10194154-arcaismos-e...

"En la Castilla rural de España, y en regiones rurales de México y países aledaños, a la mula macho y hembra se les dice "macho" y "mula" respectivamente (y en otros lugares "mula macho" y "mula hembra")."
http://es.wikipedia.org/wiki/Masculino

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Note added at 2 days2 hrs (2012-04-22 19:14:42 GMT) Post-grading
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Cheers!
Just as a little postscript to this, we had Sunday lunch today with my wife's parents, who are both from farming families in a village in La Mancha. My mother-in-law confirmed that her father had a "macho" and a "mula". They agreed that "manso" probably means "capado" (as opposed to "entero": their terms), though "machos" were not always gelded. My father-in-law's father had a "macho capado" and a "burro entero".
Mules disappeared very quickly in the 1960s when people started buying tractors.