“I've been in eight races so far this year” — talking about your results up to now.
The joke uses two tenses. The narrator tells what happened that day in the simple past. The horses and the dog talk about their results up to now — the year isn't finished — so they use the present perfect. Click ▶ to hear the sentences of the joke.
have / has + past participle
A result up to now: the period isn't finished (this year, so far), or the moment isn't important (never, ever).
I / you / we / they have ('ve) · he / she / it has ('s)
Negative: I haven't lost · Question: Have you ever seen…?
verb-ed or irregular verb
The events of the story: a finished moment in the past (one day, yesterday, last year).
Negative: he didn't lose · Question: Did he win?
✘ I have won five races last year → ✔ I won five races last year. (finished)
✔ I 've won five races this year. (not finished)