transorming spells: fey form1 and mostrous physique. getting hp from monster ?

[martn]

i just wanted to ask when im a magus, lvl 12 and have like 100hp and use one of the two spells above do i get the hp of the creature i have transformend into. for example a monster with 150 hp. i fight in that form and lose all of the hp do i turn back into my original magus form with my original 100hp. or do i only have my original hp of 100 in the form that i transformed to ?

[Greylurker]

No, all those spells do is alter your appearance, modify your stats a little and let you add some traits that the new form might have (limited by which version of the spell you use)

You still use your hit points, BAB, Saves, etc...

[Sibyl]

You "only" gain the ability listed in the spell (if the creature have them of course) and its natural attacks:
https://www.d20pfsrd.com/magic#TOC-Transmutation

[Lady Asharah]

Fey Form improves your Con so you will get +1HP/level

[Nyerkh]

No transformation in PF ever gives you the imitated creature's hp.
That's very specifically not a thing that happens, ever, and in that PF differs from d&d.
You'll often get a Con bonus however, and that's usually the extent of how your hp will be affected.

The rules have the details of what you do and do not get, as pointed out above. Let me make that into a link for you.