Konj je imao veliko kopito, a koza male rogove.

Je imao is the normal Croatian past tense (the perfect).

It is made with:

• je = auxiliary from biti (to be)
• imao = past participle of imati (to have)

So:

• konj je imao = the horse had

The participle changes to match the subject:

• konj is masculine singular → imao
• koza is feminine singular → imala

So the full second clause would be Koza je imala male rogove.

Croatian often leaves out a repeated verb when it is easy to understand from context.

So:

• full version: Konj je imao veliko kopito, a koza je imala male rogove.
• shortened version: Konj je imao veliko kopito, a koza male rogove.

English can do something similar:

• The horse had a large hoof, and the goat small horns.

It sounds a little more compressed or literary, but it is normal grammar.

Because je is a clitic, and Croatian clitics usually go in the second position of the clause.

So the neutral order is:

• Konj je imao...

This is a very common Croatian pattern:

• Ivan je došao.
• Sestra je rekla.
• Koza je imala...

To an English speaker, this can feel unusual at first, but it is one of the basic word-order rules in Croatian.

The subjects are in the nominative:

• konj = horse
• koza = goat

The things they had are direct objects, so they are in the accusative:

• kopito
• rogove

That is because imati (to have) takes a direct object in the accusative.

So structurally:

• Konj = subject
• je imao = verb
• veliko kopito = direct object

and in the second clause:

• koza = subject
• (je imala) = understood verb
• male rogove = direct object

Because kopito is:

• neuter
• singular
• here in the accusative singular

The adjective must agree with the noun in:

• gender
• number
• case

So:

• velik is the basic dictionary form
• with neuter singular, it becomes veliko

That gives:

• veliko kopito

Also, for many neuter nouns, the nominative singular and accusative singular look the same, so kopito stays kopito.

Because rogove is the accusative plural, not the nominative plural.

The noun is rog (horn). Its plural forms include:

• rogovi = nominative plural
• rogove = accusative plural

Since imati takes a direct object, Croatian uses the accusative:

• koza (je imala) male rogove

The adjective also has to match the noun, so:

• male rogove

So:

• mali rogovi = small horns as a subject
• male rogove = small horns as an object

Because the adjective has to agree with rogove in case and number.

Here rogove is:

• plural
• accusative

So the adjective also appears in the matching form:

• male

A useful thing to remember is that Croatian adjective endings often overlap. The form male can appear in more than one grammatical situation, so you should read it together with the noun it modifies.

A often means something like:

• and
• while
• whereas
• sometimes a mild but

In this sentence, it connects two contrasted pieces of information:

• Konj je imao veliko kopito, a koza male rogove.

So here it is close to:

• The horse had a large hoof, and/while the goat had small horns.

It is not exactly the same as i (and).
A often suggests a comparison or contrast between the two parts.

Croatian does not have articles like English a/an and the.

So:

• konj can mean a horse or the horse
• koza can mean a goat or the goat

The exact meaning comes from context.

If Croatian wants to be more specific, it can use other words, such as:

• taj konj = that horse
• jedan konj = one/a certain horse
• ova koza = this goat

Because that is simply how the sentence is phrased: one object is singular, the other is plural.

Croatian does not force both nouns to match in number just because they are in the same sentence. The speaker chose:

• veliko kopito = a large hoof
• male rogove = small horns

If you wanted plural hooves, you would use:

• velika kopita

So this is not a special rule of Croatian grammar; it is just the number chosen for each noun in this sentence.