hmm...could ungula really be uncula - "little hooks" here? That word could be stretched then to mean "spurs", and that makes sense as something that would be e metallo ferreo and make the horse run faster.
I'm also going to assume providebo - "provide, give" is being used with a double accusative (incorrectly, but possible if this is a Medieval poem). And I'll assume euque is a typo for eque, the vocative of equus:
1. Carry well, black horse,
carry me and hasten!
I'll give you the spurs
tender(ly), tender(ly).
2. I'll give you the spurs
of iron metal;
to help you run
I ought, I ought.
3. I'll give you the spurs,
just as gold gives back light,
for the spark entices, entices
a good horse.
In that 2nd to last line, I suspect scintilla has a double meaning, referring to both the glint of gold and the jab of the spur. Just as the glint of gold entices humans, the jab of the spur entices the horse to run. Or something...
I can't say I completely get it, and I've made some assumptions and bent some grammar to get this. Could you let us know the context, e.g. who wrote it and when?