OK - that’s because sy(dd) is present tense - it’s a special relative verb, and is the only one of its kind. So in other words, bod is unusual in having a special (present tense only!) relative form including the ‘who’. All other tenses, and all other verbs, need the particle a, which is often dropped, though the mutation isn’t.
Y fenyw sy’n dod yfory - the girl who is coming today
Y fenyw (a) fydd yn dod yfory - the girl who will come tomorrow
Y fenyw (a) ddaw yfory - the girl who will come tomorrow
Y fenyw (a) ddaeth ddoe - the girl who came yesterday